Nightmare as revolutionary spirit rises (6)
I believe the answer is:
murder
'nightmare' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'revolutionary spirit rises' is the wordplay.
'revolutionary' becomes 'red' (red can informally mean communist).
'spirit' becomes 'rum' (alcoholic spirit).
'rises' says the letters should be written backwards.
'red'+'rum'='redrum'
'redrum' in reverse letter order is 'MURDER'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for murder that I've seen before include "a lot of crows" , "Red Rum (anag)" , "Dispatch" , "very difficult task" , "hit" .)