Promoters of health spa ultimately stop (4)
I believe the answer is:
whoa
'stop' is the definition.
(command to stop a horse)
'promoters of health spa ultimately' is the wordplay.
'promoters of health' becomes 'WHO' (World Health Organisation).
'ultimately' says to take the final letters.
The final letter of 'spa' is 'a'.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Stop, Dobbin!" , "horsy expression?" , "Opposite of "Giddy-up!"" , "Command to horse to stop or stand still" , "Cry for horses to stop" .)