A one-man woman (5)
I believe the answer is:
irene
'woman' is the definition.
(I've seen this before)
'a one-man' is the wordplay.
'a one' becomes 'I' (Roman numeral).
'man' becomes 'rene' ().
'i'+'rene'='IRENE'
(Other definitions for irene that I've seen before include "In Greek mythology, the goddess of peace" , "Girl of peace" , "Girl's name (meaning Peace)" , "- - Handl, actress" , "- - Worth, actress" .)