Unusually a pretty cold old reptile (11)
I believe the answer is:
pterodactyl
'old reptile' is the definition.
'unusually a pretty cold' is the wordplay.
'unusually' is an anagram indicator.
'a'+'pretty'+'cold'='aprettycold'
'aprettycold' with letters rearranged gives 'PTERODACTYL'.
(Other definitions for pterodactyl that I've seen before include "Winged reptile of old" , "Late Jurassic flyer" , "Fossil flyer" , "Extinct flying reptile - protect lady (anag)" , "Flying creature" .)