Challenge a red revolutionary (4)
I believe the answer is:
dare
'challenge' is the definition.
(I know that dare is a type of challenge)
'a red revolutionary' is the wordplay.
'revolutionary' is an anagram indicator.
'a'+'red'='ared'
'ared' with letters rearranged gives 'DARE'.
(Other definitions for dare that I've seen before include "Dan - - , 'Eagle' pilot" , "Have the courage to try something" , "Be brave enough" , "Take a chance - read about it" , "Throw down the gauntlet" .)