Rouse tribes in revolt (6)
I believe the answer is:
bestir
'rouse' is the definition.
(I know that rouse can be written as bestir)
'tribes in revolt' is the wordplay.
'in revolt' indicates anagramming the letters.
'tribes' is an anagram of 'BESTIR'.
(Other definitions for bestir that I've seen before include "Exert/rouse (oneself)" , "Rouse (oneself)" , "Become active" , "move" .)