The girl and I have abbreviated the letter (7)
I believe the answer is:
missive
'letter' is the definition.
(synonyms)
'the girl and i have' is the wordplay.
'the girl' becomes 'miss' (term of address for an unmarried woman).
'and' is a charade indicator (letters next to each other).
'i have' becomes 'I've' (contraction of 'I have').
'miss'+'ive'='MISSIVE'
'abbreviated the' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for missive that I've seen before include "Letter" , "intelligence in this?" , "message" .)