Strength of sea creature, we hear (6)
I believe the answer is:
muscle
'strength' is the definition.
(muscle can mean strength or force)
'sea creature we hear' is the wordplay.
'sea creature' becomes 'mussel' (mussels are sea creatures).
'we hear' indicates a 'sounds like' (homophone) clue.
'mussel' sounds like 'MUSCLE'.
'of' acts as a link.
(Other definitions for muscle that I've seen before include "Bicep or tricep, for instance" , "Fibrous tissue in the body" , "meat" , "Force; a body tissue" , "Contracting body part" .)