'Not guilty'? Hammered chap's imbibed an excess! (8)
I believe the answer is:
plethora
'an excess' is the definition.
(plethora is a kind of excess)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for plethora that I've seen before include "'Large, perhaps excessive, amount (8)'" , "opposite of scarcity" , "(A) glut (of something)" , "Large number" , "Profusion" .)