Top police officers advancing caught mobster (4)
I believe the answer is:
capo
'mobster' is the definition.
(term for a mafia leader)
'top police officers advancing caught' is the wordplay.
I cannot quite see how this works, but
'top' could be 'cap' (cap is a kind of top) and 'cap' is located in the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for capo that I've seen before include "Gangster boss" , "something to grip neck tightly" , "Mafia boss" , "Leader of gang" , "crime boss" .)