Case of remarkable one-time revolutionary (8)
I believe the answer is:
reformer
'revolutionary' is the definition.
(thesaurus)
'case of remarkable one-time' is the wordplay.
'case of' suggests removing the centre (like emptying a case).
'remarkable' becomes 'rare' (I've seen this in another clue).
'one-time' becomes 'former' (I've seen this before).
'rare' with its middle taken out is 're'.
're'+'former'='REFORMER'
(Other definitions for reformer that I've seen before include "One changing for the better" , "Luther, perhaps" , "Luther, for one" , "One dealing with abuses" , "Person who advocates change" .)