Skimpy undergarment (5)
I believe the answer is:
'skimpy undergarment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thong that I've seen before include "scanty garment" , "flip-flop" , "Skimpy undergarment" , "Narrow strip of leather (or other material)" , "Narrow strip of hide used as lash" .)