Strength of mollusc escaping trap (6)

I believe the answer is:

muscle

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'strength' is the definition.
(muscle can mean strength or force)

I don't understand the remainder of the clue.

(Other definitions for muscle that I've seen before include "Physical power" , "contractor" , "Fibrous bodily tissue for strength" , "Body tissue that affects movement" , "Bicep or tricep, for instance" .)

I've seen this clue in The Guardian.

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