Large-headed nail (4)
I believe the answer is:
stud
'large-headed nail' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stud that I've seen before include "What makes shoe skidproof" , "Farm for breeding horses" , "Collar fastener or horse farm" , "Part of a football boot" , "Form of poker game" .)