Protective footwear (5)
'protective footwear' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overshoe that I've seen before include "Galosh" , "Protective item of footwear" , "which may be outside Oxford" , "Waterproof footwear" , "Re hooves? (anag)" .)
(Other definitions for boots that I've seen before include "Kicks" , "Footwear" , "shoe cleaner" .)