Immortal eastern bird on a lake (7)
I believe the answer is:
eternal
'immortal' is the definition.
(similar in meaning)
'eastern bird on a lake' is the wordplay.
'eastern' becomes 'e'.
'bird' becomes 'tern' (tern is a kind of bird).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'e'+'tern'+'a'+'l'='ETERNAL'
(Other definitions for eternal that I've seen before include "always to be remembered" , "Forever and always" , "Perpetual" , "carrying on endlessly" , "Constant" .)