True capital of Mayan kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(kingdom is a type of country or realm)
'true capital of mayan' is the wordplay.
'true' becomes 'real' (similar in meaning).
'capital of' says to take the initial letters.
The first letter of 'mayan' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Country" , "A Nation" , "Land under a king or queen" , "Kingdom could be lamer" , "Monarchy" .)