Rod connecting wheels (4)
I believe the answer is:
axle
'rod connecting wheels' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for axle that I've seen before include "Rod of wheelbase" , "Shaft on which a wheel rotates" , "Bar on which wheel revolves" , "Rotating rod carrying wheels" , "Pin on which a wheel turns" .)