Problem -- top teacher's a revolutionary (8)
I believe the answer is:
headache
'problem' is the definition.
(I've seen this before)
'top teacher's a revolutionary' is the wordplay.
'top teacher' becomes 'head'.
'revolutionary' becomes 'che' (Che Guevara the Marxist revolutionary).
'head'+'a'+'che'='HEADACHE'
(Other definitions for headache that I've seen before include "Problem, nuisance" , "big problem" , "Cause of worry/trouble" , "Pain in the cranium" , "Source of worry (and pain!)" .)