Forceful, backing a lot in police department (7)
I believe the answer is:
dynamic
'forceful' is the definition.
(similar in meaning)
'backing a lot in police department' is the wordplay.
'backing' says the letters should be written in reverse.
'a lot' becomes 'many' (many means a lot).
'in' is an insertion indicator.
'police department' becomes 'cid' (abbreviation for Criminal Investigation Department).
'many' placed within 'cid' is 'cimanyd'.
'cimanyd' back-to-front is 'DYNAMIC'.
(Other definitions for dynamic that I've seen before include "Full of energy and ideas" , "Forceful" , "High-powered" , "Constantly changing" , "Having power" .)