He can't possibly employ singer (9)
I believe the answer is:
chanteuse
'singer' is the definition.
(I've seen this before)
'he can't possibly employ' is the wordplay.
'possibly' indicates an anagram.
'employ' becomes 'use' (synonyms).
'he'+'cant'='hecant'
'hecant' with letters rearranged gives 'chante'.
'chante'+'use'='CHANTEUSE'
(Other definitions for chanteuse that I've seen before include "Female singer, in a nightclub perhaps" , "A French woman singer of popular ballads" , "Tunes ache for this French woman singer" , "French female singer in tune chase" , "She sings popular songs" .)