Waterproof boot (5)
'waterproof boot' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for wader that I've seen before include "crane, perhaps" , "High waterproof boot" , "Heron, say" , "Long-legged shore bird" , "Snipe maybe" .)
(Other definitions for welly that I've seen before include "Oomph" , "Rubber boot (abbr)" , "Boot (informal)" , "wet-weather gear" , "One going on foot" .)