Crossword Genius

Encourage son without stain to abandon east (4)

Ross

I believe the answer is:

spur

I'm a little stuck... Click here to teach me more about this clue!

'encourage' is the definition.
(spur can mean to encourage)

'son without stain to abandon east' is the wordplay.
I cannot really understand how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is located in the answer.
The remaining letters 'pur' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for spur that I've seen before include "Impetus" , "Provoke" , "Cowboy's goading instrument" , "Rider's heel wheel" , "Strong incentive" .)

I've seen this clue in The Independent.
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