Lower part of a spur? (5)

I believe the answer is:

rowel

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'part of a spur?' is the definition.
('rowel' is part of 'spur')

'lower' is the wordplay.
'lower' can be anagrammed to 'ROWEL'.
Though, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for rowel that I've seen before include "Spiked wheel on spur" , "Spur's spiked wheel" , "Goad" , "Spur spikes" , "Spiked disc on spur" .)

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