Bridle attachment (3)
I believe the answer is:
bit
'bridle attachment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bit that I've seen before include "Snaffle - small piece" , "A little coin" , "Mouthpiece of brindle or tip of drilling tool" , "The boring-piece of a drill" , "Part of 21 across" .)