Museum piece is a round cast (8)
I believe the answer is:
dinosaur
'museum piece' is the definition.
'is a round cast' is the wordplay.
'cast' is an anagram indicator (letters are dispersed or cast around).
'is'+'a'+'round'='isaround'
'isaround' with letters rearranged gives 'DINOSAUR'.
(Other definitions for dinosaur that I've seen before include "Extinct reptilian animal" , "Extinct monster like Barney, children's TV pal" , "Huge beast" , "square figure!" , "Extinct land reptile" .)