By inches, pose oddly, showing muscles (6)
I believe the answer is:
biceps
'muscles' is the definition.
(biceps are muscles)
'by inches pose oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'by'+'inches'+'pose'='byinchespose'
The alternate letters of 'byinchespose' are 'BICEPS'.
'showing' acts as a link.
(Other definitions for biceps that I've seen before include "Forearm-flexing muscle" , "Elbow-bending muscle" , "Muscle that flexes the forearm" , "I bend the elbow" , "Arm workers" .)