A revolutionary on principal source of worry (8)
I believe the answer is:
headache
'source of worry' is the definition.
(I've seen this before)
'a revolutionary on principal' is the wordplay.
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'on' means one lot of letters go next to another.
'principal' becomes 'head' ('head' can be a synonym of 'principal'**).
'a'+'che'='ache'
'ache' after 'head' is 'HEADACHE'.
(Other definitions for headache that I've seen before include "trial" , "Pain in the cranium" , "Informally a worrying problem - take an asprin" , "Severe problem" , "Problem, nuisance" .)