Forerunner's entente cad ruined (10)
I believe the answer is:
antecedent
'forerunner's' is the definition.
(synonyms)
'entente cad ruined' is the wordplay.
'ruined' indicates an anagram.
'entente'+'cad'='ententecad'
'ententecad' with letters rearranged gives 'ANTECEDENT'.
(Other definitions for antecedent that I've seen before include "Something existing before another" , "prior" , "Something previously extant" , "Previous" , "Preceding in time or order" .)