Crossword Genius

Question a command when riding? (4)

Ross

I believe the answer is:

whoa

'command when riding?' is the definition.
(command to a horse)

'question a' is the wordplay.
'question' becomes 'who' (who is a question word).
'who'+'a'='WHOA'

(Other definitions for whoa that I've seen before include "horsy expression?" , "Stop! - steady on!" , "Stop, Dobbin!" , "Opposite of "Giddy-up!"" , "The King of rock and roll" .)

I've seen this clue in The Times.
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