Fair river in China (8)
I believe the answer is:
moderate
'fair' is the definition.
(similar in meaning)
'river in china' is the wordplay.
'river' becomes 'oder' (River Oder in central Europe).
'in' indicates putting letters inside.
'china' becomes 'mate' (china plate is Cockney rhyming slang for mate).
'oder' placed within 'mate' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Restrained" , "Make reasonable" , "Average in amount" , "Medium in amount or intensity" , "Lessen the intensity" .)