Which of you has been given a command to stop? (4)
I believe the answer is:
whoa
'a command to stop?' is the definition.
(I've seen this in another clue)
'which of you has been given' is the wordplay.
'which of you' becomes 'who' (I've seen this in another clue).
'has' means one lot of letters go next to another.
'been given' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'who'+'a'='WHOA'
Can you help me to learn more?
(Other definitions for whoa that I've seen before include "Hold your horses!" , "Danger" , "Order (horse) to stop" , "Stop, Dobbin!" , "horsy expression?" .)