Fellow doctor's gagging (7)
I believe the answer is:
comedic
'gagging' is the definition.
(making gags)
'fellow doctor's' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'doctor' becomes 'medic' (doctor is a type of medic).
'co'+'medic'='COMEDIC'
(Other definitions for comedic that I've seen before include "Of humour" , "Amusing" , "Funny, laughable" , "Humorous" , "Cheerfully entertaining" .)