Tar: spoil ship with lack of litre? (7)
I believe the answer is:
mariner
'tar' is the definition.
(synonyms)
'spoil ship with lack of litre?' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'ship' becomes 'liner' (liner is a kind of ship).
'with lack of' suggests deleting specific letters.
'litre?' becomes 'l' ('l' can be a synonym of 'litre').
'liner' with 'l' removed is 'iner'.
'mar'+'iner'='MARINER'
(Other definitions for mariner that I've seen before include "'''The Rime of the Ancient ...'' (Coleridge) (7)'" , "Sea-goer, ancient in the poem" , "Sailor" , "Man of the sea" , "Bluejacket" .)