Idea that’s not finished yet (6)
I believe the answer is:
though
'yet' is the definition.
(I've seen this in another clue)
'idea that's not finished' is the wordplay.
'idea' becomes 'thought' (synonyms).
'that's not finished' means to remove the last letter.
'thought' with its final letter removed is 'THOUGH'.
(Other definitions for though that I've seen before include "Concessive conjuction" , "However" , "Even if" , "Despite the fact that" , "Nevertheless" .)