Stop doctors' adviser attending operation essentially (4)
I believe the answer is:
whoa
'stop' is the definition.
(command to stop a horse)
'doctors' adviser attending operation essentially' is the wordplay.
'doctors' adviser attending' becomes 'who' (I can't explain this - if you can you should believe this answer much more).
'essentially' indicates the central letters.
The central letter of 'operation' is 'a'.
'who'+'a'='WHOA'
Can you help me to learn more?
(Other definitions for whoa that I've seen before include "Call to stop" , "Hold your horses!" , "Command for horse to slow or stop" , "Danger" , "Said to stop a horse" .)