Police officer upset re agents (8)
I believe the answer is:
sergeant
'police officer' is the definition.
(thesaurus)
'upset re agents' is the wordplay.
'upset' indicates anagramming the letters.
're'+'agents'='reagents'
'reagents' with letters rearranged gives 'SERGEANT'.
(Other definitions for sergeant that I've seen before include "Gets near (anag) -- NCO" , "one with stripes" , "Soldier and lover in Hardy novel" , "Copper" , "Army rank" .)