Optimist panned drunken failure (14)
I believe the answer is:
disappointment
'failure' is the definition.
(I know that disappointment is a type of failure)
'optimist panned drunken' is the wordplay.
'drunken' is an anagram indicator.
'optimist'+'panned'='optimistpanned'
'optimistpanned' with letters rearranged gives 'DISAPPOINTMENT'.
(Other definitions for disappointment that I've seen before include "Let-down" , "Setback" , "unhappiness" , "Frustrating thing" , "Frustration" .)