Endless ale drunk entertaining bird (7)
I believe the answer is:
eternal
'endless' is the definition.
(I know that endless can be written as eternal)
'ale drunk entertaining bird' is the wordplay.
'drunk' indicates an anagram.
'entertaining' is an insertion indicator (inviting in, as for dinner).
'bird' becomes 'tern' (tern is a kind of bird).
'ale' with letters rearranged gives 'eal'.
'eal' placed around 'tern' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "abiding" , "Valid for all time" , "Seemingly without end" , "Everlasting, not affected by time" , "Existing forever" .)