Old fogey ruins a do, dancing (8)
I believe the answer is:
dinosaur
'old fogey' is the definition.
(both can mean an old-fashioned person)
'ruins a do dancing' is the wordplay.
'dancing' is an anagram indicator.
'ruins'+'a'+'do'='ruinsado'
'ruinsado' anagrammed gives 'DINOSAUR'.
(Other definitions for dinosaur that I've seen before include "Extinct reptile - very outdated thing" , "is well out of date" , "Extinct land reptile" , "Prehistoric monster from Jurassic era" , "Extinct terrestrial reptile" .)