Part of the Burma-Siam landmass (4)
I believe the answer is:
asia
'landmass' is the definition.
(Asia is an example)
'part of the burma-siam' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ASIA' is hidden in the letters of 'burma siam'.
(Other definitions for asia that I've seen before include "Continental land mass" , "Stretch of land" , "huge territory" , "Large continent" , "One-third of all land" .)