Vegetable from standard crop? (7)
I believe the answer is:
'vegetable' is the definition.
(parsnip is a kind of vegetable)
'standard crop?' is the wordplay.
'standard' becomes 'par' (I've seen this before).
'crop?' becomes 'snip' ('snip' can be a synonym of 'crop').
'from' acts as a link.
(Other definitions for parsnip that I've seen before include "Tapering vegetable", "Spar pin (anag) - vegetable", "Yellow root vegetable", "Plant with edible white root", "Pips ran oddly through the vegetable".)