Calm, friend crossing European river (8)
I believe the answer is:
moderate
'calm' is the definition.
(thesaurus)
'friend crossing european river' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend**).
'crossing' indicates putting letters inside.
'european river' becomes 'oder' (I've seen this before).
'mate' going around 'oder' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Become less intense" , "Not extreme - not very good" , "Medium in amount or intensity" , "Chair" , "Temper" .)