Oxygen container holding last of the water (5)
I believe the answer is:
ocean
'water' is the definition.
(ocean is a kind of water)
'oxygen container holding last of the' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'container' becomes 'can' (can is a kind of container**).
'holding' means one lot of letters goes inside another.
'last of' says to take the final letters.
The last letter of 'the' is 'e'.
'can' going around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Very large area of sea" , "Indian or Arctic, say" , "Huge body of water" , "Pacific, for example" , "One of the Seven Seas" .)