Bar on which wheels rotate (4)
I believe the answer is:
axle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bar on which wheels rotate</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for axle that I've seen before include "need of wheeled cart?" , "Rod connecting wheels" , "Wheel spindle" , "Pin in revolver" , "Mandrel" .)