Main old container touring east (5)
I believe the answer is:
ocean
'main' is the definition.
(synonyms)
'old container touring east' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'container' becomes 'can' (can is a kind of container).
'touring' is an insertion indicator (touring can mean going round).
'east' becomes 'e' (synonyms).
'can' going around 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "eg Indian, Pacific" , "One of the Seven Seas" , "Indian perhaps" , "Salt water expanse" , "Large expanse of sea" .)