Which person comes to a stop? (4)
I believe the answer is:
whoa
'stop?' is the definition.
(command to stop a horse)
'which person comes to a' is the wordplay.
'which person' becomes 'who' (I've seen this in another clue).
'comes to' means one lot of letters go next to another.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Danger" , "halt" , "Call to stop" , "Stop, Dobbin!" , "Order (horse) to stop" .)