Pay police on base (4,3,4)
I believe the answer is:
foot the bill
'pay' is the definition.
'police on base' is the wordplay.
'police' becomes 'thebill' (informal UK term for the police).
'on' means one lot of letters go next to another.
'base' becomes 'foot' ('foot' can be a synonym of 'base').
'thebill' after 'foot' is 'FOOT THE BILL'.
(Other definitions for foot the bill that I've seen before include "Pay up" , "what the not so popular music-hall entertainer must do?" , "Settle" .)