A rotten breakfast for an unpleasant fellow (3,3)
I believe the answer is:
bad egg
'fellow' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'a rotten breakfast for an unpleasant' is the wordplay.
'a rotten breakfast' becomes 'egg' (I can't explain this - if you can you should believe this answer much more).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'an unpleasant' becomes 'bad'.
'egg' put after 'bad' is 'BAD EGG'.
Can you help me to learn more?
(Other definitions for bad egg that I've seen before include "Nasty person like addled ovoid" , "Was the cartoon curate presented with one" , "Untrustworthy (a potential stinker?)" , "Worthless person" , "Rotter - something rotten!" .)