Rebels' aim in revolution is causing unhappiness (9)
I believe the answer is:
miserable
'causing unhappiness' is the definition.
(causing misery)
'rebels' aim in revolution' is the wordplay.
'in revolution' indicates an anagram.
'rebels'+'aim'='rebelsaim'
'rebelsaim' with letters rearranged gives 'MISERABLE'.
'is' acts as a link.
(Other definitions for miserable that I've seen before include "Dreary - pathetic" , "Being sad" , "Wretched, low" , "sorrowful" , "Blue" .)