Scheme that initially works (5)
I believe the answer is:
Here is my best explanation:
'works' is the definition.
('plant' can be a synonym of 'works')
'scheme that initially' is the subsidiary indication.
'scheme' becomes 'plan' (I've seen this before).
'initially' says to take the initial letters.
The initial letter of 'that' is 't'.
(Other definitions for plant that I've seen before include "workshop", "Growing organism - industrial buildings", "eg Flower", "Member of vegetable kingdom", "Shrub - factory", "Factory; informer", "Place in the ground to grow", "Put in the ground to grow", "that goes to pot" and "Snooker shot; factory".)