Bad, being active in revolution (4)
I believe the answer is:
evil
'bad' is the definition.
(I've seen this before)
'active in revolution' is the wordplay.
'active' becomes 'live' ('live' can be similar in meaning to 'active').
'in revolution' says the letters should be written in reverse.
'live' reversed gives 'EVIL'.
'being' is the link.
(Other definitions for evil that I've seen before include "Extremely bad" , "Nefarious" , "Profoundly malevolent" , "Very wicked or bad" , "Vile (anag) -- really bad" .)