Beast mother left (6)
I believe the answer is:
mammal
'beast' is the definition.
(I know that mammal is a type of craniate)
'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 "Animal that nourishes its young with milk" , "warm-blooded beast" , "It's young are nourished by milk" , "Milk-secreting animal" , "Creature like man" .)