Firm cracking frauds beat police (10)
I believe the answer is:
constables
'beat police' is the definition.
(police officers working 'on the beat')
'firm cracking frauds' is the wordplay.
'firm' becomes 'stable' (similar in meaning).
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'frauds' becomes 'cons' (con is a kind of fraud).
'stable' put inside 'cons' is 'CONSTABLES'.
(Other definitions for constables that I've seen before include "police" , "Policemen" .)