Third son of King David (7)
I believe the answer is:
absalom
'third son of king david' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for absalom that I've seen before include "King's rebellious son" , "David's boy" , "David's third son (OT)" , "Son of King David (OT)" , "Son of King David (Bible)" .)