Idea that ultimately works (5)
I believe the answer is:
plant
'works' is the definition.
(synonyms)
'idea that ultimately' is the wordplay.
'idea' becomes 'plan' (plan is a kind of idea).
'ultimately' indicates one should take the final letters.
The last letter of 'that' is 't'.
'plan'+'t'='PLANT'
(Other definitions for plant that I've seen before include "Place in the ground to grow" , "Factory; spy" , "Set; factory" , "Factory; organism" , "eg Flower" .)