1940s swinger’s cheers returned by male police officer (6)
I believe the answer is:
hepcat
'1940s swinger's' is the definition.
(term for a jazz fan)
'cheers returned by male police officer' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'returned' is a reversal indicator.
'by' is a charade indicator (letters next to each other).
'male' becomes 'he'.
'police officer' becomes 'pc' (police constable).
'ta' backwards is 'at'.
'he'+'pc'='hepc'
'at' after 'hepc' is 'HEPCAT'.
(Other definitions for hepcat that I've seen before include "Jazz or swing fan (1940s' slang)" , "one with it" , "Old jazz fan" , "trendy no longer" .)