Crossword Genius

What to do when you see red? (4)

Ross

I believe the answer is:

stop

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

'what to do when you see red?' is the definition.
(I've seen this in another clue)

'what to' is the wordplay.
I cannot really see how this works, but
'to' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for stop that I've seen before include "Go no further" , "Red light signal" , "Cease; organ voice" , "Prevent - desist" , "Part of organ" .)

I've seen this clue in the Universal and the Evening Standard.
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