Receive a royal honour after king’s left knowing nothing (9)
I believe the answer is:
benighted
'knowing nothing' is the definition.
(benighted means ignorant or unaware)
'receive a royal honour after king's left' is the wordplay.
I cannot really see how this works, but
'left' could be 'nigh' (similar in meaning) and 'nigh' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for benighted that I've seen before include "Get behind (anag) - being ignorant" , "Ignorant; caught in dark" .)