Old vessel crossing eastern sea (5)
I believe the answer is:
ocean
'sea' is the definition.
('ocean' can be a synonym of 'sea')
'old vessel crossing eastern' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'vessel' becomes 'can' (I've seen this before).
'crossing' is an insertion indicator.
'eastern' becomes 'e'.
'can' placed around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "eg Arctic" , "lot of liquid" , "Large amount of water and" , "Huge expanse" , "Huge sea" .)