Bird into ale drunkenly: it's endless (7)
I believe the answer is:
eternal
'endless' is the definition.
(I've seen this before)
'bird into ale drunkenly' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'into' is an insertion indicator.
'drunkenly' indicates an anagram.
'ale' with letters rearranged gives 'eal'.
'tern' going inside 'eal' is 'ETERNAL'.
'it's' is the link.
(Other definitions for eternal that I've seen before include "Undying" , "always to be remembered" , "Without beginning or end" , "Never failing" , "Forever or unchanging" .)