Dandy has left, a failure (4)
I believe the answer is:
flop
'a failure' is the definition.
(I know that flop is a type of failure)
'dandy has left' is the wordplay.
'dandy' becomes 'fop' (synonyms).
'has' means one lot of letters goes inside another.
'left' becomes 'l' (common abbreviation).
'fop' enclosing 'l' is 'FLOP'.
(Other definitions for flop that I've seen before include "Fiasco" , "No success" , "Collapse loosely - total failure" , "Utter failure" , "crash" .)