Old fogey ruins a do, sadly (8)
I believe the answer is:
dinosaur
'old fogey' is the definition.
(both can mean an old-fashioned person)
'ruins a do sadly' is the wordplay.
'sadly' indicates an anagram.
'ruins'+'a'+'do'='ruinsado'
'ruinsado' anagrammed gives 'DINOSAUR'.
(Other definitions for dinosaur that I've seen before include "One as T. Rex, triceratops" , "Enormous extinct reptile of ancient times" , "Beast long gone" , "type unable to adapt" , "Extinct monster like Barney, children's TV pal" .)