Toothed rack permitting movement in only one direction (7)
I believe the answer is:
ratchet
(Other definitions for ratchet that I've seen before include "One-way mechanism" , "Device forcing motion in one direction" , "Chatter about notch on wheel" , "Device consisting a toothed wheel and pawl that allows movement in one direction only" , "Mechanism with a toothed bar or wheel" .)