What to do when you see red? (4)
I believe the answer is:
stop
'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" .)