English bird beside a lake is ageless (7)
I believe the answer is:
eternal
'ageless' is the definition.
(synonyms)
'english bird beside a lake' is the wordplay.
'english' becomes 'e' (abbreviation).
'bird' becomes 'tern' (tern is a kind of bird).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'lake' becomes 'l'.
'e'+'tern'+'a'+'l'='ETERNAL'
'is' is the link.
(Other definitions for eternal that I've seen before include "Existing forever" , "Constant" , "Lasting forever, tediously persistent" , "Not stopping" , "Imperishable" .)