A below average girl (5)
I believe the answer is:
norma
'girl' is the definition.
'a below average' is the wordplay.
'below' says to put letters next to each other (in a down clue, some letters go below others).
'average' becomes 'norm' (synonyms).
'a' after 'norm' is 'NORMA'.
(Other definitions for norma that I've seen before include "name for Monroe" , "Marilyn Monroe's real first name" , "Shearer or Major" , "Opera - ex-PM's wife" , "Roman (anag) -- opera" .)