Head’s carbon copy (4)
I believe the answer is:
cape
'head's' is the definition.
(both can mean a piece of land jutting over the sea)
'carbon copy' is the wordplay.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'copy' becomes 'ape' (ape can mean to copy or imitate).
'c'+'ape'='CAPE'
(Other definitions for cape that I've seen before include "Head; garment" , "Cod possibly" , "Florida space flight centre" , "Short cloak" , "Cloak, robe" .)