Girl getting a first love letter (5)
I believe the answer is:
omega
'letter' is the definition.
(I know that omega is a type of letter)
'girl getting a first love' is the wordplay.
'girl' becomes 'meg' ().
'getting' says to put letters next to each other.
'first' means one lot of letters go next to another.
'love' becomes 'o' (love means zero in tennis).
'meg'+'a' is 'mega'.
'mega' put after 'o' is 'OMEGA'.
(Other definitions for omega that I've seen before include "Art of story-telling by body movement" , "in Greece, the limit!" , ""The Alpha and the ___"" , "letter to Greeks" , "Traditional story" .)