Film composer’s significant success (7)

I believe the answer is:

goodwin

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'film composer's' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'significant success' is the wordplay.
'significant' becomes 'good' (I can't explain this - if you can you should give a lot more credence to this answer).
'success' becomes 'win' (I've seen this in another clue).
'good'+'win'='GOODWIN'

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