Old girl rewriting manual (6)
I believe the answer is:
alumna
'old girl' is the definition.
(female graduate of a school or college)
'rewriting manual' is the wordplay.
'rewriting' is an anagram indicator.
'manual' with letters rearranged gives 'ALUMNA'.
(Other definitions for alumna that I've seen before include "She studied" , "Manual (anag) - old girl" , "Female former pupil" , "Female graduate" , "Harvard graduate" .)