A Siamese originated here (4)
I believe the answer is:
asia
'originated here' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'a siamese' is the wordplay.
'siamese' becomes 'sia' (I can't explain this - if you can you should believe this answer much more).
'a'+'sia'='ASIA'
Can you help me to learn more?
(Other definitions for asia that I've seen before include "huge amount of ground" , "lands in the east" , "Part of the world" , "Continental land mass" , "Tigers found here" .)