Old container circling English main (5)
I believe the answer is:
ocean
'main' is the definition.
('ocean' can be a synonym of 'main')
'old container circling english' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'container' becomes 'can' (can is a kind of container**).
'circling' is an insertion indicator (circling can mean going around or containing).
'english' becomes 'e' (abbreviation).
'can' enclosing 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Indian, for example" , "Large area of water" , "Salt water expanse" , "One of the Seven Seas" , "Large body of salt water" .)