Potato variety (6)
(Other definitions for yam that I've seen before include "Tropical root vegetable" , "May be a foreign potato" , "American sweet potato" , "A starchy tuber" , "Tuberous vegetable" .)
(Another definition for maris piper that I've seen is " Potato".)
'potato variety' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for russet that I've seen before include "Autumnal reddish-brown colour" , "Reddish brown; apple" , "Surest type of colour" , "Dessert apple" , "Autumnal reddish brown" .)