Horse's headgear (6)
I believe the answer is:
bridle
'horse's headgear' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bridle that I've seen before include "A curb; show scorn (at)" , "Show indignation" , "Take offence" , "a bit? That's part of it" , "Become indignant" .)