Beside himself, Jack's a fierce fellow (6)
I believe the answer is:
tartar
'a fierce fellow' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'beside himself jack's' is the wordplay.
'beside' is a charade indicator (letters next to each other).
'himself' becomes 'tar' (I can't explain this - if you can you should believe this answer much more).
'jack' becomes 'tar' (both are informal terms for sailor).
'tar' put next to 'tar' is 'TARTAR'.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "saucy stuff" , "Crust on teeth or wine deposit" , "may be hard to brush off" , "Unmanageable sort" , "plaque" .)