Thought one almost cherished (4)
I believe the answer is:
idea
'thought' is the definition.
(synonyms)
'one almost cherished' is the wordplay.
'one' becomes 'i' (Roman numeral).
'almost' means to remove the last letter (almost the whole word is kept).
'cherished' becomes 'dear' (dear can mean cherished or adored).
'dear' with its last letter taken off is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Thought or notion (4)" , "Purpose" , "Mental impression" , "Light bulb above head" , "Suggestion" .)