Pin on which a wheel turns (4)
I believe the answer is:
axle
'pin on which a wheel turns' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for axle that I've seen before include "Rod supporting wheels" , "It's pivotal" , "Rod attachment to wheels" , "Spindle between wheels" , "Pin on which a wheel turns" .)