Constrain, limit (4)
I believe the answer is:
curb
I believe this is a double definition.
'constrain' is the first definition.
(curbing is a kind of constraining)
'limit' is the second definition.
(I know that curb is a more specific form of cumber)
(Other definitions for curb that I've seen before include "[LIMITATION]" , "Rein in" , "Muzzle" , "'Temper, hold in restraint (4)'" , "Check or restrain" .)