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