Persistent bird tucked into decapitated mammal (7)
I believe the answer is:
eternal
'persistent' is the definition.
(I've seen this before)
'bird tucked into decapitated mammal' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'tucked into' is an insertion indicator.
'decapitated' means to remove the first letter.
'mammal' becomes 'seal' (type of mammal).
'seal' with its initial letter removed is 'eal'.
'tern' going within 'eal' is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Not stopping" , "Such a city is Rome" , "Immortal" , "Valid for all time" , "Forever and always" .)