Support American Revolutionary's complaint (8)
I believe the answer is:
backache
'complaint' is the definition.
(I've seen this before)
'support american revolutionary's' is the wordplay.
'support' becomes 'back' (to back is to support or endorse).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'revolutionary' becomes 'Che' (Che Guevara the Marxist revolutionary).
'back'+'a'+'che'='BACKACHE'
(Other definitions for backache that I've seen before include "Spinal pain" , "common complaint" , "Eg, lumbago" , "Pain in rear of body" , "Eg, lumbar pain" .)