Crack: cloth filling hole (3)
I believe the answer is:
gag
'crack' is the definition.
(both can mean a joke)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gag that I've seen before include "Joke; silencer" , "Stop someone telling" , "Shut up" , "Put a muzzle on the joke" , "Restriction; joke" .)