Vegetable crop is below standard (7)
I believe the answer is:
'vegetable' is the definition.
(parsnip is a kind of vegetable)
'crop is below standard' is the wordplay.
'crop' becomes 'snip' ('snip' can be a synonym of 'crop').
'is below' is a charade indicator (letters next to each other).
'standard' becomes 'par' (I've seen this before).
'snip' after 'par' is 'PARSNIP'.
(Other definitions for parsnip that I've seen before include "Tapering vegetable", "Yellow root vegetable", "Sweet root vegetable", "A white root vegetable", "Pips ran oddly through the vegetable".)