Neglected state of son adopted by dreadful couple (9)
I believe the answer is:
disrepair
'neglected state' is the definition.
(I've seen this in another clue)
'son adopted by dreadful couple' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'dreadful' becomes 'dire' (synonyms).
'couple' becomes 'pair' (synonyms).
's' put within 'dire' is 'disre'.
'disre'+'pair'='DISREPAIR'
'of' is the link.
(Other definitions for disrepair that I've seen before include "State of collapse" , "broken-down state" , "state of neglect" , "rack and ruin" , "Degradation" .)