It isn't surprising, these days, nitrogen in European river (2,6)
I believe the answer is:
no wonder
'it isn't surprising' is the definition.
(I've seen this before)
'these days nitrogen in european river' is the wordplay.
'these days' becomes 'now' (at this point in time).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'in' indicates putting letters inside.
'european river' becomes 'oder' (River Oder in central Europe).
'n' put into 'oder' is 'onder'.
'now'+'onder'='NO WONDER'
(Other definitions for no wonder that I've seen before include "hardly unforeseen" , "hardly unexpected" , "it isn't surprising" .)