Undying bird in realm's interior (7)
I believe the answer is:
eternal
'undying' is the definition.
(I've seen this before)
'bird in realm's interior' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'in' indicates putting letters inside.
'interior' says to take the centre.
The central letters of 'realm' are 'eal'.
'tern' put into 'eal' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Immortal" , "Without beginning or end" , "'Lasting forever, unending (7)'" , "Valid for all time" , "Undying" .)