Hopeful cop in decline (7)
I believe the answer is:
wannabe
'hopeful' is the definition.
(I know that hopeful can be written as wannabe)
'cop in decline' is the wordplay.
'cop' becomes 'nab' (synonyms).
'in' is an insertion indicator.
'decline' becomes 'wane' (wane is a kind of decline).
'nab' put into 'wane' is 'WANNABE'.
(Other definitions for wannabe that I've seen before include "Ambitious person" , "One ambitious to fit in, succeed" , "One aiming (ineffectively) at a certain style etc" , "One with hopes" , "Aspirant" .)