Endless ale drunk caging bird (7)
I believe the answer is:
eternal
'endless' is the definition.
(I know that endless can be written as eternal)
'ale drunk caging bird' is the wordplay.
'drunk' indicates an anagram.
'caging' indicates putting letters inside (I've seen this in other clues).
'bird' becomes 'tern' (tern is a kind of bird).
'ale' anagrammed gives 'eal'.
'eal' enclosing 'tern' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Forever or unchanging" , "Perpetual" , "Existing forever" , "Never-ending, existing always" , "Immortal" .)