Fixed part of mechanism that's wound up (6)

I believe the answer is:

mended

'fixed' is the definition.
('mend' can be a synonym of 'fix')

'part of mechanism that's wound up' is the wordplay.
'part of' indicates taking the first letters (I've seen 'part' mean this).
'that's wound up' becomes 'ended' (how things 'wind up' is how they end).
The initial letter of 'mechanism' is 'm'.
'm'+'ended'='MENDED'

(Another definition for mended that I've seen is " Repaired, fixed".)

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