Mad that one had authenticated it? (9)
I believe the answer is:
certified
'mad' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'one had authenticated it?' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'had' could be 'd' (in contractions e.g. 'I'd', 'you'd' etc) and 'd' is found within the answer.
'it?' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Other definitions for certified that I've seen before include "declared insane" , "bananas" , "Guaranteed" , "Quality-assured" .)