Monopolise department on small Scottish plant (7)
I believe the answer is:
hogweed
'plant' is the definition.
(hogweed is a kind of plant)
'monopolise department on small scottish' is the wordplay.
'monopolise' becomes 'hog' (I've seen this before).
'department' becomes 'd' (abbreviation eg in MOD Ministry of Defence).
'on' says to put letters next to each other.
'small scottish' becomes 'wee' (Scottish term for small).
'd' after 'wee' is 'weed'.
'hog'+'weed'='HOGWEED'
(Other definitions for hogweed that I've seen before include "Large white-flowered plant" , "Tall wild plant having thick stems and cluster of white to purple flowers" .)