Crazy revolutionary in communist uniform (4)
I believe the answer is:
nuts
'crazy' is the definition.
(nuts can informally mean crazy or mad)
'revolutionary in communist uniform' is the wordplay.
'revolutionary' says the letters should be written backwards (letters revolve around).
'in' indicates a hidden word.
'stun' can be found hidden inside 'communist uniform'.
'stun' in reverse letter order is 'NUTS'.
(Other definitions for nuts that I've seen before include "Unbalanced" , "Got a screw loose" , "Eccentric; coal lumps" , "balls" , "Is butter" .)