One moans a lot, but hasn't done so for very long (7)
I believe the answer is:
crybaby
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for crybaby that I've seen before include "One shedding tears frequently" , "Person readily shedding tears" , "One sheds tears easily" , "Immature weeper" , "One who weeps easily without good cause" .)