A daughter overlooked by son is out of sorts (3)
I believe the answer is:
sad
'out of sorts' is the definition.
(out of sorts can mean sad or unwell)
'a daughter overlooked by son' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'overlooked by' says to put letters next to each other (in a down clue, some letters go above others).
'son' becomes 's' (genealogical abbreviation for son).
'a'+'d'='ad'
'ad' put after 's' is 'SAD'.
'is' acts as a link.
(Other definitions for sad that I've seen before include "Dismal" , "Unfortunate" , "Despondent" , "Melancholy, triste" , "Mournful - sorry" .)