Mostly trim and cut root vegetable (7)
I believe the answer is:
'root vegetable' is the definition.
(parsnip is a kind of root vegetable)
'mostly trim and cut' is the wordplay.
'mostly' means to remove the last letter.
'trim' becomes 'pare' (I've seen this before).
'and' means one lot of letters go next to another.
'cut' becomes 'snip' (snipping is a kind of cutting).
'pare' with its final letter removed is 'par'.
(Other definitions for parsnip that I've seen before include "Yellow root vegetable", "Sweet root vegetable", "Pale carrot-like vegetable", "Plant with edible white root", "Type of root veg".)