Wheel (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">wheel</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that castor is a type of roller)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for castor that I've seen before include "Roller on chair" , "for wheel? Yes and no" , "Trolley wheel" , "Chair wheel" , "excellent once at the Gabba" .)
(Other definitions for caster that I've seen before include "Crates with kind of fine sugar" , "Founder; type of 12 Ac" , "Small swivelling wheel on furniture" , "One who throws something" , "Swivelling wheel (E, not O)" .)