Old tin cans ultimately pollute large body of water (5)
I believe the answer is:
ocean
'large body of water' is the definition.
(I've seen this in another clue)
'old tin cans ultimately pollute' is the wordplay.
'old' becomes 'o' (abbreviation).
'tin' becomes 'can' (synonyms).
'cans' means one lot of letters goes inside another (to can could mean to contain).
'ultimately' suggests the final letters.
The last letter of 'pollute' is 'e'.
'can' enclosing 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Huge sea" , "Great sea" , "Extensive sea area" , "A mighty sea" , "The deep" .)