Bitter police unit retiring agents must arrest (6)
I believe the answer is:
acidic
'bitter' is the definition.
(I've seen this in another clue)
'police unit retiring agents must arrest' is the wordplay.
'police unit' becomes 'CID' (abbreviation for Criminal Investigation Department).
'retiring' is a reversal indicator.
'agents' becomes 'CIA' (Central Intelligence Agency).
'must arrest' indicates putting letters inside.
'cia' written backwards gives 'aic'.
'cid' going inside 'aic' is 'ACIDIC'.
(Other definitions for acidic that I've seen before include "Cutting" , "Tart, sour" , "Containing a sour-tasting substance" , "Like vinegar" , "Sharp-tasting, sour" .)