Odd tuna swimming round northern river in Russia (9)
I believe the answer is:
unnatural
'odd' is the definition.
(I've seen this before)
'tuna swimming round northern river in russia' is the wordplay.
'swimming' is an anagram indicator (letters swim around into a new order).
'round' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'river in russia' becomes 'ural' (the Ural is a Russian river).
'tuna' is an anagram of 'unat'.
'unat' enclosing 'n' is 'unnat'.
'unnat'+'ural'='UNNATURAL'
(Other definitions for unnatural that I've seen before include "Perverse" , "Affected" , "odd" , "Artificial" , "Abnormal; not spontaneous" .)