Toothed rack (7)
I believe the answer is:
ratchet
'toothed rack' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ratchet that I've seen before include "Toothed rack" , "this will stop revolutionary backsliding" , "Bar that fits into the notches of a toothed wheel" , "One-way mechanism" , "Mechanism with a toothed bar or wheel" .)