A singer engaged in amorous activity (2,2)
I believe the answer is:
at it
'engaged in amorous activity' is the definition.
('at it' can mean having sex)
'a singer' is the wordplay.
'singer' becomes 'tit' (type of songbird).
'a'+'tit'='AT IT'
(Other definitions for at it that I've seen before include "working hard" , "engaged in crime?" , "Occupied (in crime?)" , "Active" , "engaged in dubious activity?" .)