Tar of wreck penetrated by saltwater, not initially (7)
I believe the answer is:
mariner
'tar' is the definition.
('mariner' can be a synonym of 'tar')
'wreck penetrated by saltwater not initially' is the wordplay.
'wreck' becomes 'mar' (I've seen this before).
'penetrated by' is an insertion indicator (I've seen this in other clues).
'saltwater' becomes 'brine' ('brine' can be a synonym of 'saltwater').
'not initially' means to remove the first letter.
'brine' with its first letter removed is 'rine'.
'mar' placed around 'rine' is 'MARINER'.
'of' is the link.
(Other definitions for mariner that I've seen before include "Man of the sea" , "Bluejacket" , "Jack" , "Seafarer, old salt" , "Seaman" .)