Frank's a hundred, with evidence to prove it (6)
I believe the answer is:
candid
'frank's' is the definition.
(synonyms)
'a hundred with evidence to prove it' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
'hundred' could be 'c' (Roman numeral) and 'c' is found within the answer.
The remaining letters 'did' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for candid that I've seen before include "Impartial" , "Honest and open about what one thinks" , "Frank and outspoken" , "Frank, honest" , "Frank - open" .)