Bird shows mother love at heart (3)
I believe the answer is:
moa
'bird' is the definition.
(extinct flightless bird of New Zealand)
'mother love at heart' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'love' becomes 'o' (love means zero in tennis).
'at heart' indicates putting letters inside (inserted letters go to the heart of the word).
'ma' enclosing 'o' is 'MOA'.
'shows' is the link.
(Other definitions for moa that I've seen before include "Flightless bird once found in New Zealand" , "Extinct flightless bird of New Zealand" , "Bird that died out" , "Extinct large flightless NZ bird" , "Gigantic extinct bird" .)