Performer who starts gagging? (5)
I believe the answer is:
comic
'performer who starts gagging?' is the definition.
(comic perform jokes or gags)
This is the entire clue.
(Other definitions for comic that I've seen before include "Funny (person, paper)" , "SECOND PART OF 16" , "Victor once, say" , "Funny; comedian" , "Children's periodical" .)