Revolutionary, ensuring changes ahead of time (9)
I believe the answer is:
insurgent
'revolutionary' is the definition.
(I've seen this before)
'ensuring changes ahead of time' is the wordplay.
'changes' indicates anagramming the letters.
'ahead of time' becomes 't' (I've seen this in other clues).
'ensuring' is an anagram of 'insurgen'.
'insurgen'+'t'='INSURGENT'
(Other definitions for insurgent that I've seen before include "Rebel, revolutionary" , "Unresting like a rebel" , "Revolting" , "freedom fighter" , "rising" .)