Limit on headwear (3)
I believe the answer is:
cap
I believe this clue is a double definition.
'limit' is the first definition.
(capping is a kind of limiting)
'headwear' is the second definition.
(cap is a type of hat)
'on' acts as a link.
(Other definitions for cap that I've seen before include "Soft hat; lid" , "Headgear item" , "contribution to hunt funds?" , "Surpass with headwear" , "Put a limit on" .)