Defeat of king's son nurtured by partner (9,4)
I believe the answer is:
smothered mate
'defeat of king's' is the definition.
(chess term)
'son nurtured by partner' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'nurtured' becomes 'mothered' (nurture can mean to mother or look after).
'by' says to put letters next to each other.
'partner' becomes 'mate' (synonyms).
's'+'mothered'+'mate'='SMOTHERED MATE'
(Other definitions for smothered mate that I've seen before include "Chess ending" , "Knight's elegant conclusion" , "No bishop can deliver one" .)