Tower of London fiend? (3,4)
I believe the answer is:
old nick
'fiend?' is the definition.
(both can mean the Devil)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for old nick that I've seen before include "reopened wound?" , "Familiar name for the devil, ancient chap" , "Scratch" , "spirit of evil" , "Newgate's" .)