Greek letter for a girl (5)
I believe the answer is:
greta
'girl' is the definition.
'greek letter' is the wordplay.
'greek' becomes 'gr' (abbreviation for Greek).
'letter' becomes 'eta' (eta is a kind of letter of the alphabet).
'gr'+'eta'='GRETA'
'for a' acts as a link.
(Other definitions for greta that I've seen before include "'I want to be alone' - ?.. Garbo (5)" , "lady" , ".....Garbo, who wanted to be alone" , "- Garbo, Swedish actress" , "Girl" .)