Unmarried girl of old (6)
I believe the answer is:
maiden
'unmarried girl of old' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for maiden that I've seen before include "First, inaugural" , "Unmated; runless over" , "Dotty over" , "No runs for the unmarried lady's over" , "Unmarried girl of old" .)