A mother? He didn't have one! (4)
I believe the answer is:
adam
'he didn't have one' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'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 "Faith, for example" , "who was tempted by mate?" , "First man in the Bible" , "the first orphan?" , "man expelled from garden" .)