Ocean of southeast Asia, primarily (3)
I believe the answer is:
sea
'ocean' is the definition.
(synonyms)
'southeast asia primarily' is the wordplay.
'southeast' becomes 'SE' (synonyms).
'primarily' indicates taking the first letters.
The first letter of 'asia' is 'a'.
'se'+'a'='SEA'
'of' is the link.
(Other definitions for sea that I've seen before include "Great expanse of water" , "Eg, Aral, Ross, Irish" , "Caspian, e.g." , "The Med, e.g." , "Watery expanse" .)