Polish detective in film (4)
I believe the answer is:
edit
'polish' is the definition.
(I've seen this in another clue)
'detective in film' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'in' means one lot of letters goes inside another.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'di' put into 'et' is 'EDIT'.
(Other definitions for edit that I've seen before include "doctor" , "Revise - expurgate" , "This is an act of cleaning" , "Be in charge of (newspaper)" , "Censor" .)