Question a command when riding? (4)
I believe the answer is:
whoa
'command when riding?' is the definition.
(command to a horse)
'question a' is the wordplay.
'question' becomes 'who' (who is a question word).
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "horsy expression?" , "Stop! - steady on!" , "Stop, Dobbin!" , "Opposite of "Giddy-up!"" , "The King of rock and roll" .)