Root vegetable (7)
I believe this is a double definition.
'root' is the first definition.
(parsnip is a kind of root)
'vegetable' is the second definition.
(parsnip is a kind of vegetable)
(Other definitions for parsnip that I've seen before include "Pips ran oddly through the vegetable", "Plant with edible white root", "Sweet root vegetable", "Type of root veg", "Spar pin (anag) - vegetable".)