Deathless seabird entering wild lea (7)
I believe the answer is:
eternal
'deathless' is the definition.
(thesaurus)
'seabird entering wild lea' is the wordplay.
'seabird' becomes 'tern' (tern is a kind of seabird).
'entering' is an insertion indicator.
'wild' indicates anagramming the letters.
'lea' anagrammed gives 'eal'.
'tern' placed into 'eal' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Undying" , "Never failing" , "'Lasting forever, unending (7)'" , "Permanent, without end" , "Seemingly without end" .)