Rodent in museum put down (8)
I believe the answer is:
musquash
'rodent' is the definition.
(musquash is a kind of rodent)
'museum put down' is the wordplay.
'museum' becomes 'mus' (abbreviation for museum).
'put down' becomes 'quash' (I've seen this before).
'mus'+'quash'='MUSQUASH'
'in' is the link.
(Other definitions for musquash that I've seen before include "Beaver-like rodent with dark glossy brown fur" , "Type of fur" , "Fur of the muskrat" , "Aquatic rodent" , "Beaver-like rodent of North America with dark glossy fur" .)