Place of mine visited by sailor (5)
I believe the answer is:
posit
'place' is the definition.
(I know that posit is a type of place)
'mine visited by sailor' is the wordplay.
'mine' becomes 'pit' (down the pit).
'visited by' means one lot of letters goes inside another.
'sailor' becomes 'os' (ordinary seaman).
'pit' enclosing 'os' is 'POSIT'.
'of' acts as a link.
(Other definitions for posit that I've seen before include "Put forward as true" , "Put forward (something) as a given" , "Lay down as principle" , "Propose" , "Put forward (as fact)" .)