Mountainous region in northeast China (5)
I believe the answer is:
nepal
'mountainous region' is the definition.
(I've seen this before)
'northeast china' is the wordplay.
'northeast' becomes 'NE' (abbreviation).
'china' becomes 'pal' (china plate is Cockney rhyming slang for mate).
'ne'+'pal'='NEPAL'
'in' acts as a link.
(Other definitions for nepal that I've seen before include "Himalayan republic" , "mountainous region" , "Country located mainly in the Himalayas" , "Mountainous country" , "Country in plane north-west of India" .)