Aggressive colonist's like rebel when disturbed (6,3)
I believe the answer is:
killer bee
'aggressive colonist's' is the definition.
(bees live in colonies)
'like rebel when disturbed' is the wordplay.
'when disturbed' indicates an anagram.
'like'+'rebel'='likerebel'
'likerebel' is an anagram of 'KILLER BEE'.
(Other definitions for killer bee that I've seen before include "Aggressive type" , "Aggressive insect" , "Honey-producer" .)