Man, say, left supporting mother (6)
I believe the answer is:
mammal
'man say' is the definition.
(man is a kind of mammal)
'left supporting mother' is the wordplay.
'left' becomes 'L' (common abbreviation).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'mother' becomes 'mamma' (mamma is a kind of mother).
'l' put after 'mamma' is 'MAMMAL'.
(Other definitions for mammal that I've seen before include "Animal of a class giving birth to live young" , "Animal that nourishes its young with milk" , "Creature like man" , "Milk-secreting animal" , "Suckling animal" .)