Simple earring (4)
I believe the answer is:
stud
'simple earring' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stud that I've seen before include "What makes shoe skidproof" , "Horse-breeding farm" , "Collar button or animal farm" , "Clothes fastener in horse establishment" , "collection of cars" .)