Device allowing motion in one direction only (7)
I believe the answer is:
ratchet
'device allowing motion in one direction only' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ratchet that I've seen before include "Bar that fits into the notches of a toothed wheel" , "Bar to fit the notches of a toothed wheel" , "Chatter (anag)" , "Chatter about notch on wheel" , "Toothed rack" .)