In cars, they're mostly forward (5)
I believe the answer is:
gears
This clue could be a double definition.
'in cars' is the first definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Maybe there's an association between them I don't understand?
'mostly forward' is the second definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'they're' is the link.
Can you help me to learn more?
(Other definitions for gears that I've seen before include "Rages (anag.)" , "Sarge (anag.)" .)