Plant, factory bringing in one attached to Foreign Office (7)
I believe the answer is:
milfoil
'plant' is the definition.
(milfoil is a kind of plant)
'factory bringing in one attached to foreign office' is the wordplay.
'factory' becomes 'mill' ('mill' can be a synonym of 'factory').
'bringing in' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'attached to' means one lot of letters go next to another.
'foreign office' becomes 'fo'.
'i' after 'fo' is 'foi'.
'mill' placed around 'foi' is 'MILFOIL'.
(Another definition for milfoil that I've seen is " Common wasteland plant - film oil (anag)".)