Bird into ale sloshed: it's endless (7)
I believe the answer is:
eternal
'endless' is the definition.
(I know that endless can be written as eternal)
'bird into ale sloshed' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'into' indicates putting letters inside.
'sloshed' is an anagram indicator.
'ale' anagrammed gives 'eal'.
'tern' put within 'eal' is 'ETERNAL'.
'it's' is the link.
(Other definitions for eternal that I've seen before include "Forever and always" , "Existing forever" , "Lasting forever, tediously persistent" , "Valid for all time" , "Without beginning or end" .)