Bit attachment (4)
I believe the answer is:
rein
'bit attachment' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rein that I've seen before include "Restraining lead" , "Controlling strap" , "Restraint; strap attached to horse's bit" , "Harness item" , "one's attached to a bit" .)