Brutal fellow, cur ultimately hostile in former colony (7)
I believe the answer is:
gorilla
'brutal fellow' is the definition.
'cur ultimately hostile in former colony' is the wordplay.
'ultimately' says to take the final letters.
'hostile' becomes 'ill' ('ill' can be similar in meaning to 'hostile').
'in' indicates putting letters inside.
'former colony' becomes 'goa'.
The final letter of 'cur' is 'r'.
'r'+'ill'='rill'
'rill' placed into 'goa' is 'GORILLA'.
(Other definitions for gorilla that I've seen before include "Large primate" , "Big ape" , "Largest living primate" , "Aggressive-looking type" , "Powerful great ape" .)