Girl, excellent female, big back (5)
I believe the answer is:
sofia
'girl' is the definition.
'excellent female big back' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'female' becomes 'f' (abbreviation).
'big' becomes 'os'.
'back' is a reversal indicator.
'ai'+'f'+'os'='aifos'
'aifos' back-to-front is 'SOFIA'.
(Other definitions for sofia that I've seen before include "Capital of Bulgaria" , "Bulgarian capital" , "capital city" , "Balkan capital" , "Foreign city" .)