Mother left small animals (7)
I believe the answer is:
mammals
'animals' is the definition.
(mammal is a kind of animal)
'mother left small' is the wordplay.
'mother' becomes 'mamma' (mamma is a kind of mother).
'left' becomes 'L' (common abbreviation).
'small' becomes 's' (abbreviation - e.g. clothes size).
'mamma'+'l'+'s'='MAMMALS'
(Other definitions for mammals that I've seen before include "Animals that suckle their young" , "Warm-blooded vertebrates" , "Warm-blooded vertebrate animals" , "Animals nourished with mother's milk" , "Class of animals" .)