Spur disc (5)
I believe the answer is:
rowel
'spur disc' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rowel that I've seen before include "Spur's spiked wheel" , "sort of wheel" , "piece on Spurs" , "Part of a spur" , "Goad" .)