Narcos providing support for one proposal (4)
I believe the answer is:
idea
'proposal' is the definition.
(I've seen this before)
'narcos providing support for one' is the wordplay.
'narcos' becomes 'dea' (Drug Enforcement Agency, informally called narcos).
'providing support for' means one lot of letters go next to another (in a down clue, some letters go below others).
'one' becomes 'i' (roman numeral).
'dea' put after 'i' is 'IDEA'.
(Other definitions for idea that I've seen before include "Thought; concept" , "Notion in the mind" , "Aim" , "Clue" , "Notion, belief" .)