Short answer: yes — never mistake slow for contained
A Clumber Spaniel does need a securely fenced yard, even though he is nobody's idea of an escape athlete. The Clumber is the largest and heaviest of the spaniel breeds, a long, low, massively built gundog developed for working dense cover slowly and thoroughly. He will not bounce over your fence or sprint through a closing gate. What he will do is follow a scent with the patience of a dog bred to quarter thick undergrowth all day, leaning his considerable weight against a weak board, nosing under a gate, or shuffling through a gap in the hedge that nobody considered a real opening.
With a Clumber, containment fails quietly and slowly rather than dramatically — which makes it easy to underestimate until the morning he is simply not in the yard.
How a Clumber actually gets out
Scent is the engine. Clumbers work close and methodically, and a trail that leads under or through the fence invites exactly the steady investigation the breed was built for. Food sharpens the motivation: this is a notoriously food-focused breed, and a neighbor's barbecue, fallen fruit, or accessible bins on the other side of the line are powerful arguments to a Clumber's way of thinking.
Strength is the tool. A mature Clumber carries surprising mass on a low frame, and steady pressure from that body loosens aging boards and widens soft spots at the base of a fence over time. He may also dig modestly after scent, especially where animals have burrowed along the fence line. None of this requires speed — it requires only an unsupervised hour and a fence that was almost good enough.
Fencing suited to a heavy spaniel
Prioritize solidity at dog height and below. Panels should be firm enough to take a big dog's lean without flexing, the bottom edge should sit close to the ground along the whole run, and gates need latches that hold against persistent nosing and shoulder pressure. Walk the line after wet weather, when soil softens at the base and a determined dog can scrape a workable hollow in one session.
Height demands are modest — a standard fence a heavy, non-jumping dog cannot scramble over is sufficient — but remove step-ups near the perimeter, since a Clumber will happily use a woodpile or planter as a vantage point and, occasionally, as a route. Keep the yard itself interesting and the dog adequately exercised; Clumbers are calmer than most spaniels, yet an under-stimulated one with a working nose will assign himself projects, and the fence line is where the interesting smells live.
The wider safety picture
Clumbers are gentle, devoted dogs who generally prefer their people, so escapes are usually scent-led wanderings rather than bids for freedom — but a slow dog on a road is in as much danger as a fast one, and a large white dog ambling through traffic is depending entirely on drivers noticing him. Supervise yard time rather than leaving him out for hours, keep his weight managed so he stays mobile and healthy, and maintain a recall even if it is a leisurely one.
Identification is the final layer for the day persistence wins, and that is where KinsLe's Lost Pet Alert System adds real help, sending a free alert to nearby Circle of Love members so a finder can message you directly.