Idea that initially works (5)
I believe the answer is:
plant
'works' is the definition.
(synonyms)
'idea that initially' is the wordplay.
'idea' becomes 'plan' (plan is a kind of idea).
'initially' indicates taking the first letters.
The first letter of 'that' is 't'.
'plan'+'t'='PLANT'
(Other definitions for plant that I've seen before include "Factory; informer" , "Set; factory" , "Living organism without locomotion" , "Spy; manufacturer's site" , "Place in the ground to grow" .)