Energetic backing for detectives arresting large numbers (7)
I believe the answer is:
dynamic
'energetic' is the definition.
('dynamic' is associated in meaning with 'energetic')
'backing for detectives arresting large numbers' is the wordplay.
'backing for' is a reversal indicator.
'detectives' becomes 'cid'.
'arresting' indicates putting letters inside.
'large numbers' becomes 'many'.
'cid' enclosing 'many' is 'cimanyd'.
'cimanyd' backwards is 'DYNAMIC'.
(Other definitions for dynamic that I've seen before include "Full of energy and ideas" , "Vigorous and effective" , "Energetic, with great force of character" , "Vigorous and go-ahead" , "I'm Candy (anag) - full of beans" .)