Open shoe (6)
'open shoe' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sandal that I've seen before include "Wood" , "Openwork shoe" , "Type of footwear" , "Item of footwear" , "One can go on foot" .)
(Other definitions for flip-flop that I've seen before include "Light shoe" , "Informal footwear" , "Shoe held on by a toe-thong" , "Flimsy sandal" , "Rubber sandal" .)