Silly senate needs to be reformed substantially (11)
I believe the answer is:
essentially
'substantially' is the definition.
(thesaurus)
'silly senate needs to be reformed' is the wordplay.
'needs to be reformed' indicates anagramming the letters.
'silly'+'senate'='sillysenate'
'sillysenate' is an anagram of 'ESSENTIALLY'.
(Other definitions for essentially that I've seen before include "Fundamentally" , "mainly" , "Primarily" , "By its very nature" , "Basically" .)