Betting on a new bridge (4)
I believe the answer is:
span
'bridge' is the definition.
(I know that bridge can be written as span)
'betting on a new' is the wordplay.
'betting' becomes 'sp'.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'sp'+'a'+'n'='SPAN'
(Other definitions for span that I've seen before include "Width of pans" , "Distance between two points in time or place" , "period of time" , "Part of bridge, nine inches" , "Extent, space for odd pans" .)