Ultimate decider of a police departmental match? (4,4)
I believe the answer is:
acid test
'ultimate decider' is the definition.
'a police departmental match?' is the wordplay.
'police' becomes 'cid' (abbreviation for Criminal Investigation Department).
'departmental match?' becomes 'test' (test match is a kind of match. I am not sure about the 'departmental' bit.).
'a'+'cid'+'test'='ACID TEST'
'of' is the link.
(Other definitions for acid test that I've seen before include "Crucial assessment" , "what may prove conclusive" , "A severe or conclusive exam or trial" , "Ultimate proof" , "Conclusive moment (using litmus paper?)" .)