Letter from a girl I've got attached to (7)
I believe the answer is:
missive
'letter' is the definition.
(I know that letter can be written as missive)
'a girl i've got attached to' is the wordplay.
'a girl' becomes 'miss' (term of address for an unmarried woman).
'got attached to' is a charade indicator (letters next to each other).
'miss'+'ive'='MISSIVE'
'from' is the link.
(Other definitions for missive that I've seen before include "intelligence in this?" , "message" , "Letter" .)