Through seawater ploughed giant ship (10)
I believe the answer is:
brigantine
'ship' is the definition.
(I know that brigantine is a type of sailing ship)
'through seawater ploughed giant' is the wordplay.
'through' means one lot of letters goes inside another (some letters go through others).
'seawater' becomes 'brine' (synonyms).
'ploughed' is an anagram indicator.
'giant' with letters rearranged gives 'ganti'.
'brine' placed around 'ganti' is 'BRIGANTINE'.
(Other definitions for brigantine that I've seen before include "Two-masted sailing ship" , "vessel" , "Boat" .)