Overturn the failure of a thong (4-4)
I believe the answer is:
flip-flop
'a thong' is the definition.
(I know that flip-flop can be written as flip-flop)
'overturn the failure' is the wordplay.
'overturn' becomes 'flip' (I've seen this in another clue).
'the failure' becomes 'flop' (flop is a kind of failure).
'flip'+'flop'='FLIP-FLOP'
'of' is the link.
(Other definitions for flip-flop that I've seen before include "Shoe held on by a toe-thong" , "Informal footwear" , "Light shoe" , "Sandal held on by a toe-thong" , "Rubber sandal" .)