Left a dandy out as a failure (4)
I believe the answer is:
flop
'a failure' is the definition.
(flop is a kind of failure)
'left a dandy out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'a dandy' becomes 'fop' ('fop' can be a synonym of 'dandy').
'out' means one lot of letters goes inside another.
'l' inserted within 'fop' is 'FLOP'.
'as' is the link.
(Other definitions for flop that I've seen before include "Fiasco" , "Failure - loll" , "Total failure; fall heavily" , "Dud - bomb" , "Fail" .)