Extensive evidence of debt hidden by police officers (7)
I believe the answer is:
copious
'extensive' is the definition.
(synonyms)
'evidence of debt hidden by police officers' is the wordplay.
'evidence of debt' becomes 'iou' (an IOU, 'I owe you').
'hidden by' is an insertion indicator (inserted letters are hidden inside).
'police officers' becomes 'cops' (cop is a kind of police officer).
'iou' placed into 'cops' is 'COPIOUS'.
(Other definitions for copious that I've seen before include "Plentiful, existing in large amounts" , "full" , "Rich" , "Providing loads of" , "great supply?" .)