Restrained friend crossing European river (8)
I believe the answer is:
moderate
'restrained' is the definition.
(I've seen this before)
'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' (River Oder in central Europe).
'mate' going around 'oder' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Slacken" , "Become less intense" , "So-so; sensible" , "Temper" , "Average, middle rate" .)