Generous policeman promises to pay (7)
I believe the answer is:
copious
'generous' is the definition.
(thesaurus)
'policeman promises to pay' is the wordplay.
'policeman' becomes 'cop' (I've seen this before).
'promises to pay' becomes 'ious' (more than one iou).
'cop'+'ious'='COPIOUS'
(Other definitions for copious that I've seen before include "Abundant in supply" , "Providing loads of" , "Many" , "Large in number or quantity" , "Plentiful, existing in large amounts" .)