One almost expensive thought (4)
I believe the answer is:
idea
'thought' is the definition.
('idea' can be a synonym of 'thought')
'one almost expensive' is the wordplay.
'one' becomes 'i' (Roman numeral).
'almost' means to remove the last letter (most of the word but not all of it).
'expensive' becomes 'dear' (I've seen this before).
'dear' with its last letter removed is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Inkling" , "Creative spark" , "Thought or notion (4)" , "A suspicion of" , "Proposal" .)