Frontrunners of wild horses obeying a request to stop (4)
I believe the answer is:
whoa
'request to stop' is the definition.
'frontrunners of wild horses obeying a' is the wordplay.
'frontrunners of' suggests taking the first letters.
The initial letters of 'wild horses obeying' is 'who'.
'who'+'a'='WHOA'
(Other definitions for whoa that I've seen before include "Danger" , "horsy expression?" , "Said to stop a horse" , "Call to stop" , "Command for horse to slow or stop" .)