Crossword Genius

Which of you came to a stop? (4)

Ross

I believe the answer is:

whoa

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

'which of you came' is the wordplay.
'which of you' becomes 'who' (I've seen this in another clue).
'came' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'who'+'a'='WHOA'

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(Other definitions for whoa that I've seen before include "Instruction to a horse to stop" , "Opposite of "Giddy-up!"" , "halt" , "Command to horse to stop or stand still" , "Stop! - steady on!" .)

I've seen this clue in the Evening Standard.
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