Smother crack (3)
I believe the answer is:
gag
I believe this clue is a double definition.
'smother' is the first definition.
(thesaurus)
'crack' is the second definition.
(both can mean a joke)
(Other definitions for gag that I've seen before include "Muffle with slang joke" , "Joke or muffler - a palindrome" , "Humorous anecdote or remark" , "Joke; smother" , "Silence the joke" .)