South-east Asia's principal ocean (3)
I believe the answer is:
sea
'ocean' is the definition.
(I know that ocean can be written as sea)
'south-east asia's principal' is the wordplay.
'south east' becomes 'SE' (abbreviation).
'principal' says to take the initial letters (the principal letter is the first letter).
The first letter of 'asia' is 'a'.
'se'+'a'='SEA'
(Other definitions for sea that I've seen before include "& 14 Ozone" , "The drink" , "The Aegean, for example" , "one may be pounding the beach" , "Division of an ocean" .)