Halter attachment (4)
I believe the answer is:
rein
'halter 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 "A means of control" , "Director's power" , "Part of a horses's harness" , "Restriction" , "Leather used for control in horse-riding" .)