Reported bivalve’s strength (6)
I believe the answer is:
muscle
'strength' is the definition.
(muscle can mean strength or force)
'reported bivalve's' is the wordplay.
'reported' indicates a 'sounds like' (homophone) clue.
'bivalve' becomes 'mussel' (mussel is a kind of bivalve).
'mussel' is a homophone of 'MUSCLE'.
(Other definitions for muscle that I've seen before include "Physical power" , "Bicep or tricep, for instance" , "Fibrous bodily tissue for strength" , "It can contract in the body" , "for bearing around?" .)