Vegetable from standard crop? (7)
I believe the answer is:
parsnip
'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').
'par'+'snip'='PARSNIP'
'from' acts as a link.
(Other definitions for parsnip that I've seen before include "Type of root veg" , "A white root vegetable" , "Yellow root vegetable" , "Pale carrot-like vegetable" , "Pips ran oddly through the vegetable" .)