Doctor comes to a halt (4)
I believe the answer is:
whoa
'halt' is the definition.
(I've seen this before)
'doctor comes to a' is the wordplay.
'doctor' becomes 'who' (I have seen 'Fictional doctor' mean 'who' so perhaps 'doctor' could also mean 'who').
'comes to' says to put letters next to each other.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "horsy expression?" , "Stop, Dobbin!" , "Opposite of "Giddy-up!"" , "Instruction to a horse to stop" , "Order (horse) to stop" .)