Jack's double is a fearsome fellow (6)
I believe the answer is:
tartar
'a fearsome fellow' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's a link between them I don't understand?
'jack's double' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'double' indicates the doubling of some letters.
'tar' doubled is 'TARTAR'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "may be hard to brush off" , "Incrustation on teeth and gums" , "ancient Mongolian" , "Sauce often accompanying fish" , "Fearsome person: kind of sauce" .)