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' (common abbreviation eg in OE for Old English).
'tin' becomes 'can' (synonyms).
'cans' is an insertion indicator (to can could mean to contain).
'ultimately' says to take the final letters.
The final letter of 'pollute' is 'e'.
'can' going around '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" .)