Odds on a new bridge (4)
I believe the answer is:
span
'bridge' is the definition.
(I know that bridge can be written as span)
'odds on a new' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' says to put 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 "Extent of bridge or life" , "Extent from end to end or across" , "Extent of aircraft's wing" , "Extend across, bridge" , "Distance between two points in time or place" .)