Deathless seabird enters wild lea (7)
I believe the answer is:
'deathless' is the definition.
'seabird enters wild lea' is the wordplay.
'seabird' becomes 'tern' (tern is a kind of seabird).
'enters' means one lot of letters goes inside another.
'wild' indicates anagramming the letters.
'lea' is an anagram of 'eal'.
'tern' placed within 'eal' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Timeless" , "One being forever" , "Not stopping" , "with lasting benefit" , "Forever or unchanging" .)