Animal has mother over 50 (6)
I believe the answer is:
mammal
'animal' is the definition.
(mammal is a kind of animal)
'mother over 50' is the wordplay.
'mother' becomes 'mamma' (mamma is a kind of mother**).
'over' means one lot of letters go next to another.
'a0' becomes 'l' (roman numeral).
'mamma'+'l'='MAMMAL'
'has' is the link.
(Other definitions for mammal that I've seen before include "Live-young animal" , "Whale, for example" , "Creature like man" , "Animal of a class giving birth to live young" , "One of a class of warm-blooded vertebrates" .)