Discourage power-crazy revolutionary (4)
I believe the answer is:
damp
'discourage' is the definition.
(thesaurus)
'power-crazy revolutionary' is the wordplay.
'power' becomes 'p' (symbol in physics).
'crazy' becomes 'mad' (synonyms).
'revolutionary' shows that the letters should be reversed in order (letters revolve around).
'p'+'mad'='pmad'
'pmad' reversed gives 'DAMP'.
(Other definitions for damp that I've seen before include "Rising -, TV sitcom" , "soggy" , "Clammy" , "Somewhat moist" , "Moisture" .)