Waters in the southeast part of Asia (4)
I believe the answer is:
seas
'waters' is the definition.
(sea is a kind of water)
'in the southeast part of asia' is the wordplay.
'in the' is an insertion indicator.
'southeast' becomes 'ses' (I am not sure about this - if you are sure you should believe this answer much more).
'part of' suggests taking the first letters (I've seen 'part' mean this).
The first letter of 'asia' is 'a'.
'ses' going around 'a' is 'SEAS'.
Can you help me to learn more?
(Other definitions for seas that I've seen before include "Oceans" , "Red and Dead" , "Bodies of salt water" , "Aegean and Mediterranean, say" , "Red and Black are" .)