To be mad not having succeeded is foolish (5)
I believe the answer is:
inane
'foolish' is the definition.
(similar in meaning)
'to be mad not having succeeded' is the wordplay.
'to be mad' becomes 'insane' (to be mad is to be insane).
'not having' is a deletion indicator.
'succeeded' becomes 's' (genealogical abbreviation).
'insane' with 's' taken out is 'INANE'.
'is' is the link.
(Other definitions for inane that I've seen before include "Foolish, silly" , "Stupid and silly" , "Empty-headed" , "Frivolous" , "Silly or senseless" .)