Wheel (6)
'wheel' is the definition.
(I know that castor is a type of roller)
This is the entire clue.
(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)" .)