Bishop wittering on, forgetting Welsh is incomprehensible (8)
I believe the answer is:
baffling
'incomprehensible' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for baffling that I've seen before include "Mysterious" , "Mystifying" , "Perplexing, puzzling" , "Confusing, puzzling" , "Hard to understand" .)