Medium uniform given to very short rebel (6)
I believe the answer is:
mutiny
'rebel' is the definition.
(I know that mutiny is a type of rebel)
'medium uniform given to very short' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'given to' says to put letters next to each other.
'very short' becomes 'tiny' (tiny means very small).
'm'+'u'+'tiny'='MUTINY'
(Other definitions for mutiny that I've seen before include "Open rebellion against constituted authority" , "Revolt against authority" , "Insurrection" , "Rising, rebellion" , "Military rebellion" .)