Notable prisoner accepting prison, ultimately, is revolutionary (6)
I believe the answer is:
signal
'notable' is the definition.
(thesaurus)
'prisoner accepting prison ultimately is revolutionary' is the wordplay.
'prisoner' becomes 'lag' (I've seen this before**).
'accepting' indicates putting letters inside.
'ultimately' indicates one should take the final letters.
'revolutionary' is a reversal indicator.
The final letter of 'prison' is 'n'.
'lag' enclosing 'n' is 'lang'.
'lang'+'is'='langis'
'langis' back-to-front is 'SIGNAL'.
(Other definitions for signal that I've seen before include "Gesture, motion" , "Non-verbal action with an encoded message" , "Aligns (anag) -- gesture" , "Message in the form of a gesture" , "Striking; gesture" .)