Best part of missive from firm girl (7)
I believe the answer is:
colette
'missive from firm girl' is the definition.
'Colette' can be an answer for 'girl'. I am not sure about the 'missive from firm' bit.
'best part' is the wordplay.
'best' becomes 'colet' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'part' becomes 'te' (te is a kind of part).
'colet'+'te'='COLETTE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for colette that I've seen before include "French writer" , "French novelist, author of Gigi, d. 1954" , "Fr. novelist, wrote Gigi" , "French female novelist" .)