Pinion's partner (4)
I believe the answer is:
rack
'pinion's partner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rack that I've seen before include "Cause of anguish" , "Instrument of torture, stretched its victims" , "Old instrument of torture" , "Rail for torture" , "Driving mist" .)