Peaceful state west of Mongolia (4)
I believe the answer is:
calm
'peaceful' is the definition.
(similar in meaning)
'state west of mongolia' is the wordplay.
'state' becomes 'cal' (short for California).
'west of' suggests taking the first letters (most west - leftmost - letter).
The initial letter of 'mongolia' is 'm'.
'cal'+'m'='CALM'
(Other definitions for calm that I've seen before include "Self-possessed" , "Windless" , "'After a storm comes a ....'" , "Peaceful, not excited" , "Cool, not excited" .)