Single sweetheart doesn't finish proposal (4)
I believe the answer is:
idea
'proposal' is the definition.
(idea can mean a proposal or suggestion)
'single sweetheart doesn't finish' is the wordplay.
'single' becomes 'I' (one).
'sweetheart' becomes 'dear' (both are terms of address to romantic partner).
'doesn't finish' means to remove the last letter.
'dear' with its last letter taken off is 'dea'.
'i'+'dea'='IDEA'
(Other definitions for idea that I've seen before include "Theory" , "Notion, concept" , "Brainwave" , "Thought or notion (4)" , "Something imagined in the mind" .)