Proposal not on, about to go belly-up (5)
I believe the answer is:
offer
'proposal' is the definition.
(an offer is a proposal or suggestion)
'not on about to go belly-up' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'about' becomes 're' (regarding).
'to go belly up' shows that the letters should be reversed in order (I've seen 'gone belly up' mean this (in a down clue, letters go up)).
're' written backwards gives 'er'.
'off'+'er'='OFFER'
(Other definitions for offer that I've seen before include "Volunteer, present" , "Tender for acceptance" , "Tender, volunteer" , "Present for acceptance or rejection" , "Submit" .)