Mother left animal (6)
I believe the answer is:
mammal
'animal' is the definition.
(mammal is a kind of animal)
'mother left' is the wordplay.
'mother' becomes 'mamma' (mamma is a kind of mother).
'left' becomes 'l' (common abbreviation).
'mamma'+'l'='MAMMAL'
(Other definitions for mammal that I've seen before include "Milk-secreting animal" , ".Animal of class that suckles its young" , "Animal of a class giving birth to live young" , "Whale, for example" , "warm-blooded beast" .)