Margaret 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)
'margaret getting a first love' is the wordplay.
'margaret' becomes 'Meg' (short for Margaret).
'getting' says to put letters next to each other.
'first' is a charade indicator (letters next to each other).
'love' becomes 'o' (love means zero in tennis).
'meg'+'a' is 'mega'.
'mega' after 'o' is 'OMEGA'.
(Other definitions for omega that I've seen before include "Last letter of the Greek alphabet" , "symbol of resistance?" , "letter from abroad" , "Art of story-telling by body movement" , "Ending of a series or sequence" .)