Failing Soho supremo? (7)

I believe the answer is:

sinking

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'failing' is the definition.
(I've seen this before)

'soho supremo?' is the wordplay.
'soho' becomes 'sin' (I can't explain this - if you can you should believe this answer much more).
'supremo?' becomes 'king' (both can refer to a person with great authority).
'sin'+'king'='SINKING'

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(Other definitions for sinking that I've seen before include "Declining" , "Falling to the bottom" , "Going down" , "failing" , "in decline" .)

I've seen this clue in The Independent.

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