Musical composition in film posh editor sent up (5)
I believe the answer is:
etude
'musical composition' is the definition.
(I know that etude is a type of musical composition)
'film posh editor sent up' is the wordplay.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'editor' becomes 'ed' (abbreviation).
'sent up' shows that the letters should be reversed in order.
'ed' in reverse letter order is 'de'.
'et'+'u'+'de'='ETUDE'
'in' is the link.
(Other definitions for etude that I've seen before include "Testing musical composition" , "(Musical) study" , "Indian statesman" , "A short musical exercise" , "Short piece of music for solo instrument" .)