One mostly loved thought (4)
I believe the answer is:
idea
'thought' is the definition.
(synonyms)
'one mostly loved' is the wordplay.
'one' becomes 'i' (roman numeral).
'mostly' means to remove the last letter.
'loved' becomes 'dear' (similar in meaning).
'dear' with its last letter taken away is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Plan" , "intellectual product" , "Creative spark" , "Notion, concept" , "a wheeze" .)