Sailor Jack is a fierce fellow! (6)
I believe the answer is:
tartar
'a fierce fellow' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'sailor jack' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'jack' becomes 'tar' (both are informal terms for sailor).
'tar'+'tar'='TARTAR'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "Cream of - - : baking ingredient" , "Formidable person" , "This sauce often goes with fish" , "saucy stuff" , "plaque" .)