Comic holding in good ad-lib (4,2)
I believe the answer is:
wing it
'ad-lib' is the definition.
(I've seen this in another clue)
'comic holding in good' is the wordplay.
'comic' becomes 'wit' (a witty person).
'holding' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'in'+'g'='ing'
'wit' enclosing 'ing' is 'WING IT'.
(Other definitions for wing it that I've seen before include "Improvise" , "Perform without any preparation" .)