Fiendish source of many a bad omen (5)
I believe the answer is:
demon
'fiendish source' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'many a bad omen' is the wordplay.
'many' becomes 'd' ().
'a bad' indicates an anagram.
'omen' anagrammed gives 'emon'.
'd'+'emon'='DEMON'
'of' is the link.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Evil spirit, devil" , "Wizard" , "See 3" , "Skilful person" , "malignant spirit" .)