True leader of mountainous kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(thesaurus)
'true leader of mountainous' is the wordplay.
'true' becomes 'real' ('real' can be similar in meaning to 'true').
'leader of' suggests taking the first letters.
The initial letter of 'mountainous' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "field of study" , "Kingdom's" , "where ruler holds sway?" , "Dominion" , "Kingdom could be lamer" .)