Thought one loved, dropping resistance (4)
I believe the answer is:
idea
'thought' is the definition.
(I know that thought can be written as idea)
'one loved dropping resistance' is the wordplay.
'one' becomes 'i' (Roman numeral).
'loved' becomes 'dear' (similar in meaning).
'dropping' is a deletion indicator.
'resistance' becomes 'r' (electronics abbreviation).
'dear' with 'r' taken out is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Creative spark" , "Suggestion" , "Aim" , "Thought, belief" , "Brainwave" .)