Gloat unfairly over the three that flopped completely (10)
I believe the answer is:
altogether
'completely' is the definition.
(synonyms)
'gloat unfairly over the three that flopped' is the wordplay.
'unfairly' indicates anagramming the letters (I've seen this in other clues).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'that flopped' indicates anagramming the letters (I've seen 'flop' mean this (a flop is a mistake or poorly executed action)).
'gloat' anagrammed gives 'altog'.
'three' anagrammed gives 'ether'.
'altog'+'ether'='ALTOGETHER'
(Other definitions for altogether that I've seen before include "Completely, on the whole" , "Wholly" , "nude in this" , "totally" , "Entirely" .)