Limits on headwear (4)
I believe the answer is:
caps
This clue is a double definition.
'limits' is the first definition.
(capping is a kind of limiting)
'headwear' is the second definition.
'on' is the link.
(Other definitions for caps that I've seen before include "Coverings, for heads say" , "Style of GBS writing" , "is better" , "Imposes limits on" , "Crowns" .)