Strip of leather (5)
'strip of leather' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thong that I've seen before include "G-string" , "Leather strip that forms the flexible part of a whip: excuse for underwear" , "Strip of leather used for fastening" , "Thin leather strip" , "Narrow strip of leather (or other material)" .)
(Other definitions for strap that I've seen before include "Fastening strip" , "Leather fastening" , "punishment in school" , "Leather thong" , "Whip" .)