Endure sweetheart holding up proposal (4)
I believe the answer is:
bide
'endure' is the definition.
(I've seen this in another clue)
'sweetheart holding up proposal' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'holding up' is a charade indicator (letters next to each other) (some letters go underneath or hold up others).
'proposal' becomes 'bid' (a bid is a proposal or offer).
'e' put after 'bid' is 'BIDE'.
(Other definitions for bide that I've seen before include "Dwell, of old" , "Stay in a certain place" , "To - one's time is to wait for the right opportunity" , "Tarry" , "Remain or stay (poetically)" .)