If a girl has one left, it's of some use (5)
I believe the answer is:
avail
'if a girl has one left' is the definition.
I can't tell whether this defines the answer.
'of some use' is the wordplay.
'of some' indicates the central letters (I've seen 'some' mean this).
'use' becomes 'have available' (having available is a kind of using).
The middle of 'haveavailable' is 'AVAIL'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for avail that I've seen before include "Be of value to" , "Provide help; profit" , "Medal (informal)" , "Serve a purpose" , "Use to one's advantage" .)
