Where own reflection might appear furious (6,7)
I believe the answer is:
beside oneself
'furious' is the definition.
(beside oneself can mean very angry)
I can't explain the rest of the clue.
(Other definitions for beside oneself that I've seen before include "Lost control, distracted" , "Overcome with worry" , "Irrational, eg with worry" , "Distraught" , "mad" .)