Endless bats enter area next to lake (7)
I believe the answer is:
eternal
'endless' is the definition.
(I know that endless can be written as eternal)
'bats enter area next to lake' is the wordplay.
'bats' indicates anagramming the letters (slang for mad).
'area' becomes 'a' (maths abbreviation).
'next to' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'enter' is an anagram of 'etern'.
'etern'+'a'+'l'='ETERNAL'
(Other definitions for eternal that I've seen before include "Undying" , "Valid for all time" , "with lasting benefit" , "always to be remembered" , "abiding" .)