Stop question and answer (4)
I believe the answer is:
whoa
'stop' is the definition.
(command to stop a horse)
'question and answer' is the wordplay.
'question' becomes 'who' (who is a question word).
'and' means one lot of letters go next to another.
'answer' becomes 'a' (abbreviation eg in Q and A).
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Opposite of "Giddy-up!"" , "Hold your horses!" , "Call to stop" , "Command for horse to slow or stop" , "Cry for horses to stop" .)