A mother without whom none of us would exist! (4)
I believe the answer is:
adam
'without whom none of us would exist' is the definition.
I can't judge whether this definition defines the answer.
'a mother' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'a'+'dam'='ADAM'
Can you help me to learn more?
(Other definitions for adam that I've seen before include "- - Bede by George Eliot" , "first in the race?" , "The first known ancestor" , "Early man" , "Man on the ceiling of the Sistine Chapel" .)