Calm river in China (8)
I believe the answer is:
moderate
'calm' is the definition.
(thesaurus)
'river in china' is the wordplay.
'river' becomes 'oder' (Oder is an example**).
'in' means one lot of letters goes inside another.
'china' becomes 'mate' ('China plate' is rhyming slang for 'mate').
'oder' inserted within 'mate' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Not excessively right- or left-wing" , "Read tome (anag) - unexceptional" , "Middle-of-the-road, not extreme" , "Mild" , "Become less extreme" .)