Crook, awful fiend in public, with excessive conviction (13)
I believe the answer is:
overconfident
'with excessive conviction' is the definition.
(with too much confidence or conviction)
'crook awful fiend in public' is the wordplay.
'crook' becomes 'con'.
'awful fiend' becomes 'fiden' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'public' becomes 'overt' (both can mean well-known or unconcealed).
'con'+'fiden'='confiden'
'confiden' going within 'overt' is 'OVERCONFIDENT'.
Can you help me to learn more?
(Other definitions for overconfident that I've seen before include "Cock-sure" , "Too sure of oneself" , "Too certain of oneself" , "Too bold or certain" , "Too bullish" .)