Check river in China (8)
I believe the answer is:
moderate
'check' is the definition.
(I know that check can be written as moderate)
'river in china' is the wordplay.
'river' becomes 'oder' (River Oder in central Europe).
'in' is an insertion indicator.
'china' becomes 'mate' (china plate is Cockney rhyming slang for mate).
'oder' going within 'mate' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Average; make less extreme" , "Average; not radical" , "Ordinary" , "Average, middle rate" , "Not extreme - not very good" .)