Substantial old event is revived (9)
I believe the answer is:
extensive
'substantial' is the definition.
(thesaurus)
'old event is revived' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'revived' indicates anagramming the letters (I've seen this in other clues).
'event'+'is'='eventis'
'eventis' anagrammed gives 'tensive'.
'ex'+'tensive'='EXTENSIVE'
(Other definitions for extensive that I've seen before include "terribly large" , "Covering much area" , "Comprehensive" , "Widespread" , "Covering a large area" .)