Man in with the writer is a composer (7)
I believe the answer is:
rossini
'composer' is the definition.
(Italian composer Gioachino Rossini)
'man in with the writer' is the wordplay.
'man' becomes 'ross' ().
'with' means one lot of letters go next to another.
'the writer' becomes 'I' (the person who is writing these clues).
'ross'+'in'+'i'='ROSSINI'
'is a' acts as a link.
(Other definitions for rossini that I've seen before include "Composer born 29 Feb 1792" , "opera producer" , "Composer of The Thieving Magpie" , "Largo al factotum composer" , "Italian scorer" .)