Bird in muck on state borders (5)
I believe the answer is:
goose
'bird' is the definition.
(goose is a kind of bird)
'muck on state borders' is the wordplay.
'muck' becomes 'goo' (synonyms).
'on' says to put letters next to each other.
'borders' says to hollow out the word (remove centre letters) (only the edge or border letters remain).
'state' with its centre removed is 'se'.
'goo'+'se'='GOOSE'
'in' acts as a link.
(Other definitions for goose that I've seen before include "Supposedly foolish bird" , "Gander's mate" , "Large waterbird" , "Web-footed long-necked bird" , "Seasonal fare" .)