One who delivers middle class mother's children, at last (7)
I believe the answer is:
milkman
'one who' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'middle class mother's children at last' is the wordplay.
'middle' becomes 'M' (abbreviation for middle).
'class' becomes 'ilk' (both can mean a type or variety).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'at last' indicates one should take the final letters.
The final letter of 'children' is 'n'.
'm'+'ilk'+'ma'+'n'='MILKMAN'
'delivers' is the link.
Can you help me to learn more?
(Other definitions for milkman that I've seen before include "Deliverer of dairy products" , "Morning deliverer" , "Man in charge of a float" , "Pinta deliverer" , "Float driver" .)