Wild acclaim if top becomes a foregone conclusion (4,8)
I believe the answer is:
fait accompli
'a foregone conclusion' is the definition.
(both can mean something already decided)
'wild acclaim if top' is the wordplay.
'wild' indicates an anagram.
'acclaim'+'if'+'top'='acclaimiftop'
'acclaimiftop' with letters rearranged gives 'FAIT ACCOMPLI'.
'becomes' acts as a link.
(Other definitions for fait accompli that I've seen before include "Something irreversibly decided" , "Already done, unalterable" , "Something done and unalterable" , "Irreversible action" , "A done deal" .)