He's about as worthy as a bishop, sir (6)
I believe the answer is:
knight
'sir' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for knight that I've seen before include "Give title to; chess piece" , "Chevalier" , "Man on board" , "Chesspiece (in shining armour?)" , "Gawain?" .)