Support very large gangster's suggestion (8)
I believe the answer is:
'suggestion' is the definition.
(I've seen this in another clue)
'support very large gangster's' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'very large' becomes 'os' (abbreviation for oversize).
'gangster' becomes 'al' (Al Capone).
(Other definitions for proposal that I've seen before include "Kind of offer, perhaps of marriage" , "a match might be arranged?" , "Plan, suggestion" , "Offer (to get spliced?)" , "union offer?" .)