Membrane unusually scaly? OK ... (4-3)

I believe the answer is:

yolk-sac

'membrane' is the definition.

'unusually scaly? ok' is the wordplay.
'unusually' is an anagram indicator.
'scaly'+'ok'='scalyok'
'scalyok' is an anagram of 'YOLK-SAC'.

I've seen this clue in The Everyman puzzle in The Observer.

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