Like a mother bird swallowed by heartless mammal (8)
I believe the answer is:
maternal
'like a mother' is the definition.
I can't judge whether this defines the answer.
'bird swallowed by heartless mammal' is the wordplay.
'bird' becomes 'tern' (tern is a kind of bird).
'swallowed by' means one lot of letters goes inside another.
'heartless' suggests removing the centre.
'mammal' with its centre removed is 'maal'.
'tern' placed inside 'maal' is 'MATERNAL'.
Can you help me to learn more?
(Other definitions for maternal that I've seen before include "Motherly" , "On the mother's side" , "Protective" , "Of a mother or motherhood" .)