Maybe you can finally, you said, have children? Not so! (6)
I believe the answer is:
eunuch
'not so' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'maybe you can finally you said have children?' is the wordplay.
'finally' suggests the final letters.
'you said' becomes 'U' (homophone of 'you').
'have' says to put letters next to each other.
'children?' becomes 'ch' (genealogical abbreviation).
The final letters of 'maybe you can' are 'eun'.
'eun'+'u'+'ch'='EUNUCH'
(Other definitions for eunuch that I've seen before include "Powerless type" , "Castrated man" , "Ineffective man" , "Feeble person" , "Ineffectual man" .)