Part of a spur (5)
I believe the answer is:
'part of a spur' is the definition.
('rowel' is part of 'spur')
This is the entire clue.
(Other definitions for rowel that I've seen before include "piece on Spurs" , "Lower into a spur disc" , "Spur spikes" , "Part of a spur" , "Spur's spiked disc" .)