Compound put up an old girl (6)
I believe the answer is:
alumna
'old girl' is the definition.
(female graduate of a school or college)
'compound put up an' is the wordplay.
'compound' becomes 'alum' (chemical compound).
'put up' shows that the letters should be reversed in order.
'an' back-to-front is 'na'.
'alum'+'na'='ALUMNA'
(Other definitions for alumna that I've seen before include "Harvard graduate" , "She studied" , "Manual (anag) - old girl" , "Female graduate" , "Female former pupil" .)