Cross when entertaining character at the end repeated gag (6)
I believe the answer is:
muzzle
'gag' is the definition.
(synonyms)
'cross when entertaining character at the end repeated' is the wordplay.
'cross' becomes 'mule' (I've seen this in other clues).
'when entertaining' is an insertion indicator.
'character at the end' becomes 'z' (letter at the end of the alphabet).
'repeated' shows that letters should be duplicated.
'z' duplicated is 'zz'.
'mule' placed around 'zz' is 'MUZZLE'.
(Other definitions for muzzle that I've seen before include "Dog's jaw; to silence" , "Dog's jaws and nose" , "Snout - front end of a gun barrel" , "Device to prevent a dog biting" , "Open end of a gun barrel" .)