Stop TV doctor beginning to amputate (4)
I believe the answer is:
whoa
'stop' is the definition.
(I've seen this before)
'tv doctor beginning to amputate' is the wordplay.
'tv doctor' becomes 'who' (I've seen this in other clues).
'beginning to' says to take the initial letters.
The first letter of 'amputate' is 'a'.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Command to horse to stop or stand still" , "The King of rock and roll" , "Danger" , "Said to stop a horse" , "Call to stop" .)