Proverb used by Conrad, a genius (5)
I believe the answer is:
adage
'proverb' is the definition.
(synonyms)
'used by conrad a genius' is the wordplay.
'used by' says the answer is hidden in the clue.
'ADAGE' is hidden within 'conrad a genius'.
(Other definitions for adage that I've seen before include "Proverb, maxim" , "Old saying, maxim" , "True proverb or saying" , "Dictum" , "Proverb expressing a general truth" .)