A thousand, you said? Very small rebellion (6)
I believe the answer is:
mutiny
'rebellion' is the definition.
(I know that mutiny is a type of rebellion)
'a thousand you said? very small' is the wordplay.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'you said?' becomes 'U' (homophone of 'you').
'very small' becomes 'tiny' (I've seen this before).
'm'+'u'+'tiny'='MUTINY'
(Other definitions for mutiny that I've seen before include "Rising, rebellion" , "Open rebellion by sailors or soldiers" , "Insurrection" , "Rebellion as on Bligh's Bounty" , "Open rebellion against constituted authority" .)