Thailand's capital — capital backing Asian language (5)
I believe the answer is:
tamil
'asian language' is the definition.
(I've seen this before)
'thailand's capital capital backing' is the wordplay.
'capital' suggests taking the first letters.
'capital' becomes 'lima' (capital of Peru).
'backing' says the letters should be written backwards.
The initial letter of 'thailand' is 't'.
'lima' written backwards gives 'amil'.
't'+'amil'='TAMIL'
(Other definitions for tamil that I've seen before include "Sri Lankan" , "South Indian people or language" , "Dravidian people of India and Sri Lanka" , "Member of South Indian people" , "S. India/Sri Lankan person, tongue" .)