Decline to come round to arrest impersonator? (7)
I believe the answer is:
wannabe
'impersonator?' is the definition.
(someone who wants to be like someone else)
'decline to come round to arrest' is the wordplay.
'decline' becomes 'wane' (wane is a kind of decline).
'to come round' is an insertion indicator.
'to arrest' becomes 'nab' (synonyms).
'wane' going around 'nab' is 'WANNABE'.
(Other definitions for wannabe that I've seen before include "Aspiring film star possibly" , "One ambitious to fit in, succeed" , "Ambitious person" , "hopeful" , "One with hopes" .)