Serous membrane that covers abdominal organs (10)

I believe the answer is:

peritoneum

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'serous membrane' is the definition.
(peritoneum is a kind of serous membrane)

I cannot understand how the remainder of the clue works.

(Another definition for peritoneum that I've seen is " Membrane lining the abdominal cavity".)

I've seen this clue in The Times.

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