Sailor twice produces battle-axe (6)
I believe the answer is:
tartar
'battle-axe' is the definition.
(a harsh, fierce person)
'sailor twice' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'twice' shows that letters should be duplicated.
'tar' duplicated is 'TARTAR'.
'produces' is the link.
(Other definitions for tartar that I've seen before include "Fearsome person: kind of sauce" , "saucy stuff" , "Formidable person" , "A sauce to go with fish" , "Crust on teeth or wine deposit" .)