Sign initially the union may fold (7)

I believe the answer is:

omentum

'fold' is the definition.
(a fold of peritoneum in the human body)

'sign initially the union may' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign**).
'initially' indicates taking the first letters.
The initial letters of 'the union may' is 'tum'.
'omen'+'tum'='OMENTUM'

(Another definition for omentum that I've seen is " Fold".)

I've seen this clue in The Independent.

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