Swimmer in northern city upset about article (4,3)
I believe the answer is:
sand eel
'swimmer' is the definition.
(sand eel is a kind of fish)
'northern city upset about article' is the wordplay.
'northern city' becomes 'leeds'.
'upset' says the letters should be written backwards.
'about' indicates putting letters inside.
'article' becomes 'an' (indefinite article).
'leeds' in reverse letter order is 'sdeel'.
'sdeel' enclosing 'an' is 'SAND-EEL'.
'in' acts as a link.
(Other definitions for sand eel that I've seen before include "Very small fish that burrows into beaches" , "Shallow-water fish" , "swimmer" .)