Work supported by sure winner in firm (8)
I believe the answer is:
concerto
'work' is the definition.
(I've seen this before)
'supported by sure winner in firm' is the wordplay.
'supported by' becomes 'on' (something on something else is supported by it).
'sure winner' becomes 'cert'.
'in' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'on'+'cert'='oncert'
'oncert' placed into 'co' is 'CONCERTO'.
(Other definitions for concerto that I've seen before include "Composition for accompanied soloist" , "Orchestral work for solo instrument" , "piece of music" , "Composition for orchestra and soloist" , "Piece for soloist and orchestra" .)