Earliest vampire bats seen by lake (8)
I believe the answer is:
primeval
'earliest' is the definition.
(I've seen this in another clue)
'vampire bats seen by lake' is the wordplay.
'bats' is an anagram indicator (slang for mad).
'seen by' says to put 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 "Instinctive" , "Prehistoric" , "Ancient" , "Belonging to the Earth's beginnings" , "having come first in history" .)