Jack, once more, is 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 they are linked in a way I don't understand?
'jack once more' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'once more' becomes 'tar' (I can't explain this - if you can you should believe this answer much more).
'tar'+'tar'='TARTAR'
'is' is the link.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "A sauce to go with fish" , "Hard, crusty deposit on the teeth" , "Sauce often accompanying fish" , "Deposit that forms on teeth" , "Unpleasant sort" .)