Separated himself — at heart is a rogue (4)
I believe the answer is:
heel
'rogue' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for heel that I've seen before include "Untrustworthy person" , "One of the crusty ends of a loaf" , "See 1 down" , "Spur" , "Call to dog" .)