Mother has love at heart for a bird (3)
I believe the answer is:
moa
'bird' is the definition.
(extinct flightless bird of New Zealand)
'mother has love' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'has' indicates putting letters inside.
'love' becomes 'o' (love means zero in tennis).
'ma' placed around 'o' is 'MOA'.
'at heart for a' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for moa that I've seen before include "Bird no longer" , "Bird that died out" , "was hunted by Maori" , "Old bird" , "Bird's gone for ever" .)