Ancient Byzantine vampire near lake (8)
I believe the answer is:
primeval
'ancient' is the definition.
(I've seen this before)
'byzantine vampire near lake' is the wordplay.
'byzantine' indicates an anagram (byzantine means complex or confusing).
'near' is a charade indicator (letters next to each other).
'lake' becomes 'l' (geographical abbreviation).
'vampire' with letters rearranged gives 'primeva'.
'primeva'+'l'='PRIMEVAL'
(Other definitions for primeval that I've seen before include "From the earliest times" , "Belonging to the Earth's beginnings" , "Very old" , "Instinctive" , "Ancient" .)