Horse with hardened skin on part of ear (7)
I believe the answer is:
corncob
'part of ear' is the definition.
(as in an ear of corn)
'horse with hardened skin' is the wordplay.
'horse' becomes 'cob' (type of horse).
'with' says to put letters next to each other.
'hardened skin' becomes 'corn' (as in corns on one's feet).
'cob' put after 'corn' is 'CORNCOB'.
'on' is the link.
(Other definitions for corncob that I've seen before include "Woody centre of an ear of maize" , "Woody part of maize ear" , "Head of maize" , "Maize ear used for tobacco pipe" , "Source of a yellow vegetable" .)