Cunning gags aren't very good (7)
I believe the answer is:
saintly
'very good' is the definition.
'cunning gags aren't' is the wordplay.
'cunning' becomes 'sly' ('sly' can be a synonym of 'cunning').
'gags' is an insertion indicator.
'aren't' becomes 'aint'.
'sly' placed around 'aint' is 'SAINTLY'.
(Other definitions for saintly that I've seen before include "good!" , "Very virtuous" , "Very holy" , "Beatific" .)