Seaman repeatedly seen as fearsome person (6)
I believe the answer is:
tartar
'fearsome person' is the definition.
(I've seen this before)
'seaman repeatedly' is the wordplay.
'seaman' becomes 'tar' ('tar' can be a synonym of 'seaman').
'repeatedly' shows that letters should be duplicated.
'tar' duplicated is 'TARTAR'.
'seen as' acts as a link.
(Other definitions for tartar that I've seen before include "A ferocious person" , "Hard, crusty deposit on the teeth" , "Dental deposit" , "Crust on teeth or wine deposit" , "may be hard to brush off" .)