Old vessel crossing eastern body of water (5)
I believe the answer is:
ocean
'body of water' is the definition.
(ocean is a kind of body of water)
'old vessel crossing eastern' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'vessel' becomes 'can' (type of container).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'eastern' becomes 'e'.
'can' placed around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Vast quantity of liquid" , "Sea - canoe (anag)" , "eg Indian, Atlantic" , "Indian or Arctic, say" , "The deep" .)