Crossword Genius

Which individual gets a command to stop? (4)

Ross

I believe the answer is:

whoa

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

'command to stop?' is the definition.
(I've seen this in another clue)

'which individual gets a' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
The remaining letters 'who' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for whoa that I've seen before include "The King of rock and roll" , "Command to horse to stop or stand still" , "Command for horse to slow or stop" , "horsy expression?" , "Call to stop" .)

I've seen this clue in The Times.
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