Incompetent fool, the old man, mostly supported by a daughter (7)
I believe the answer is:
fathead
'incompetent' is the definition.
(both can mean stupid person)
'fool the old man mostly supported by a daughter' is the wordplay.
'fool' becomes 'f' (this could be a standard abbreviation which I don't know about).
'old man' becomes 'da' (da can mean dad).
'mostly' means to remove the first letter.
'supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'daughter' becomes 'd' (genealogical abbreviation).
'da' with its first letter removed is 'a'.
'the'+'a'='thea'
'thea' after 'a' is 'athea'.
'f'+'athea'+'d'='FATHEAD'
Can you help me to learn more?
(Other definitions for fathead that I've seen before include "Fool" , "Slang idiot with enlarged cranium?" , "silly ass!" , "Stupid person" , "Dimwit" .)