Revolutionary or reasonable man (4)
I believe the answer is:
rook
'man' is the definition.
(man can mean a chess piece)
'revolutionary or reasonable' is the wordplay.
'revolutionary' says the letters should be written in reverse.
'reasonable' becomes 'ok'.
'or' backwards is 'ro'.
'ro'+'ok'='ROOK'
(Other definitions for rook that I've seen before include "flier" , "Fleece - bird" , "Bird; playing piece" , "Gregarious black bird" , "Ringing in the ears" .)