Limits on headgear (4)
I believe the answer is:
caps
This clue is a double definition.
'limits' is the first definition.
(capping is a kind of limiting)
'headgear' is the second definition.
(caps are a type of headgear)
'on' acts as a link.
(Other definitions for caps that I've seen before include "Bottle tops" , "Headgear for internationals" , "Coverings for heads or bottles" , "Style of GBS writing" , "is better" .)