Ancient vampire bats left (8)
I believe the answer is:
'ancient' is the definition.
(I've seen this in another clue)
'vampire bats left' is the wordplay.
'bats' is an anagram indicator (slang for mad).
'left' becomes 'l' (common abbreviation).
'vampire' anagrammed gives 'primeva'.
(Other definitions for primeval that I've seen before include "Very old" , "Instinctive" , "From the earliest times" , "having come first in history" , "Ancient" .)