Mean to cut a vegetable (7)
I believe the answer is:
'a vegetable' is the definition.
(parsnip is a type of vegetable)
'mean to cut' is the wordplay.
'mean' becomes 'par' (average score on a golf course).
'to cut' becomes 'snip' (snipping is a kind of cutting).
(Other definitions for parsnip that I've seen before include "Yellow root vegetable" , "Spar pin (anag) - vegetable" , "Plant with edible white root" , "Pips ran oddly through the vegetable" , "Sweet root vegetable" .)