Greek character leading insignificant rebellion (6)
I believe the answer is:
mutiny
'rebellion' is the definition.
(mutiny is a kind of rebellion)
'greek character leading insignificant' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'leading' is a charade indicator (letters next to each other).
'insignificant' becomes 'tiny' (I've seen this before).
'mu'+'tiny'='MUTINY'
(Other definitions for mutiny that I've seen before include "Revolt against authority" , "Insurrection" , "Rising, rebellion" , "Rebellion in the armed forces" , "Open rebellion by sailors or soldiers" .)