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 "Put in the ground to grow", "Living organism without locomotion", "eg Flower", "Shrub - factory", "Vegetable organism", "Set; factory", "Growing organism - industrial buildings", "Industrial machinery or large vehicles", "One placed in a group as an informer" and "Member of vegetable kingdom".)