Don't split small dead plant (7)
I believe the answer is:
hogweed
'plant' is the definition.
(hogweed is a kind of plant)
'don't split small dead' is the wordplay.
'don't split' becomes 'hog' (if you hog something you refuse to share).
'small' becomes 'wee' (Scottish term for small).
'dead' becomes 'd' (abbreviation in government records).
'hog'+'wee'+'d'='HOGWEED'
(Other definitions for hogweed that I've seen before include "Tall wild plant having thick stems and cluster of white to purple flowers" , "Large white-flowered plant" .)