Two seafarers producing battle-axe (6)
I believe the answer is:
tartar
'battle-axe' is the definition.
(a harsh, fierce person)
'two seafarers' is the wordplay.
'two seafarers' can mean two substitutions for 'seafarer'.
'seafarer' becomes 'tar' (synonyms).
'seafarer' becomes 'tar' (synonyms).
'tar'+'tar'='TARTAR'
'producing' acts as a link.
(Other definitions for tartar that I've seen before include "may be hard to brush off" , "This sauce often goes with fish" , "ancient Mongolian" , "Sauce often accompanying fish" , "She terrifies" .)