United Nations support rebellion (6)
I believe the answer is:
unrest
'rebellion' is the definition.
(I've seen this before)
'united nations support' is the wordplay.
'united nations' becomes 'un' ('UN' can be a synonym of 'united nations').
'support' becomes 'rest' (rest is a kind of support**).
'un'+'rest'='UNREST'
(Other definitions for unrest that I've seen before include "State of agitation" , "'Disturbance, agitation (6)'" , "Rebellious state" , "Disturbance or dissatisfaction" , "Discontent, disturbance" .)