Middle of wooden object rejected as material for hammer (4)
I believe the answer is:
bone
'material for hammer' is the definition.
(the hammer is a bone of the ear)
'middle of wooden object rejected' is the wordplay.
'middle of' says the answer is hidden in the clue (the answer is somewhere in the middle of the clue).
'rejected' says the letters should be written backwards.
'enob' is hidden in the letters of 'wooden object'.
'enob' backwards is 'BONE'.
'as' is the link.
(Other definitions for bone that I've seen before include "Skeleton substance" , "It makes up the skeleton" , "#Close to it is risque" , "Carpal or tarsal, for example" , "Ossified material" .)