Tuna swimming around northern river? Weird (9)
I believe the answer is:
unnatural
'weird' is the definition.
(thesaurus)
'tuna swimming around northern river?' is the wordplay.
'swimming' is an anagram indicator.
'around' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'river?' becomes 'ural' (I've seen this before**).
'tuna' with letters rearranged gives 'unat'.
'unat' placed around 'n' is 'unnat'.
'unnat'+'ural'='UNNATURAL'
(Other definitions for unnatural that I've seen before include "Affected" , "Monstrous" , "Artificial" , "Freakish" , "Lacking normal feelings of, say, affection" .)