Pain commander inflicted on a revolutionary (8)
I believe the answer is:
headache
'pain' is the definition.
(I know that headache is a type of pain)
'commander inflicted on a revolutionary' is the wordplay.
'commander' becomes 'head' (I've seen this in another clue).
'inflicted on' says to put letters next to each other (in a down clue, some letters go on top of others).
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'head'+'a'+'che'='HEADACHE'
(Other definitions for headache that I've seen before include "A pain in the brain" , "Problem, nuisance" , "Source of worry (and pain!)" , "Major problem" , "Pain in the cranium" .)