A diplomat or a doctor seen to fool another about nothing (10)
I believe the answer is:
ambassador
'a diplomat' is the definition.
(ambassador is a kind of diplomat)
'or a doctor seen to fool another about nothing' is the wordplay.
I cannot really understand how this works, but
'or' is within the answer.
'doctor' could be 'mb' (Bachelor of Medicine, Medicinae Baccalaureus) and 'mb' is located in the answer.
'fool' could be 'ass' (both can mean a stupid person) and 'ass' is found within the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is found within the answer.
The remaining letters 'ad' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ambassador that I've seen before include "Overseas diplomat" , "In a different case, he could be" , "delegate" , "High-ranking diplomat" , "diplomatic type" .)