Possibly new companion, a girl (5)
I believe the answer is:
wench
'a girl' is the definition.
(I know that wench is a type of girl)
'possibly new companion' is the wordplay.
'possibly' is an anagram indicator.
'companion' becomes 'ch' (Companion of Honour).
'new' is an anagram of 'wen'.
'wen'+'ch'='WENCH'
(Other definitions for wench that I've seen before include "Servant girl" , "(Young) woman" , "An archaic girl" .)