Formidable person seen with a couple of sailors (6)
I believe the answer is:
tartar
'formidable person' is the definition.
(I've seen this before)
'with a couple of sailors' is the wordplay.
'a couple of sailors' means two substitutions for 'sailor'.
'with' says to put letters next to each other.
'sailor' becomes 'tar' (term for a sailor).
'sailor' becomes 'tar' (term for a sailor).
'tar' put next to 'tar' is 'TARTAR'.
'seen' is the link.
(Other definitions for tartar that I've seen before include "A sauce to go with fish" , "A ferocious person" , "Fearsome person: kind of sauce" , "She terrifies" , "Hard, crusty deposit on the teeth" .)