Immortal seabird in unkempt lea (7)
I believe the answer is:
eternal
'immortal' is the definition.
(similar in meaning)
'seabird in unkempt lea' is the wordplay.
'seabird' becomes 'tern' (tern is a kind of seabird).
'in' indicates putting letters inside.
'unkempt' is an anagram indicator.
'lea' anagrammed gives 'eal'.
'tern' put into 'eal' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "unchanged by time" , "always to be remembered" , "carrying on endlessly" , "Timeless" , "Existing forever" .)