Cogwheel (4)
I believe the answer is:
gear
'cogwheel' is the definition.
(synonyms)
This is all the clue.
(Other definitions for gear that I've seen before include "Trendy clothes - changed in car?" , "Toothed wheel that interlocks with another" , "working order" , "Set of wheels working by engaging cogs" , "often changed when driving" .)