To fool a fellow, you must have credibility (8)
I believe the answer is:
convince
'you must have credibility' is the definition.
I can't judge whether this definition defines the answer.
'to fool a fellow' is the wordplay.
'to fool' becomes 'con' (I've seen this in another clue).
'a fellow' becomes 'Vince' (man's name).
'con'+'vince'='CONVINCE'
Can you help me to learn more?
(Other definitions for convince that I've seen before include "Make believe" , "Satisfy as to truth" , "Satisfy by evidence" , "Persuade, satisfy" , "Persuade someone of the truth of something" .)