Earliest vampire destroyed fifty (8)
I believe the answer is:
primeval
'earliest' is the definition.
(I've seen this in another clue)
'vampire destroyed fifty' is the wordplay.
'destroyed' indicates anagramming the letters.
'fifty' becomes 'L' (Roman numeral).
'vampire' anagrammed gives 'primeva'.
'primeva'+'l'='PRIMEVAL'
(Other definitions for primeval that I've seen before include "Prehistoric" , "Very old" , "Instinctive" , "Ancient" , "From the earliest times" .)