Fidelio's finale, fluid composition (4)

I believe the answer is:

opus

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'composition' is the definition.
('opus' can be a synonym of 'composition')

'fidelio's finale fluid' is the wordplay.
'finale' indicates one should take the final letters.
'fluid' becomes 'pus' (I can't explain this - if you can you should believe this answer much more).
The last letter of 'fidelio' is 'o'.
'o'+'pus'='OPUS'

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(Other definitions for opus that I've seen before include "Creative work, eg in writing, art or music" , "Soup (anag.)" , "Piece of music (with number)" , "Musical piece" , "Artistic work" .)

I've seen this clue in The Telegraph.

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