Arranges backing in races, e.g. at Sandown (6)
I believe the answer is:
stages
'arranges' is the definition.
('stage' can be a synonym of 'arrange')
'backing in races e g at sandown' is the wordplay.
'backing' is a reversal indicator.
'in' indicates a hidden word.
'segats' can be found hidden inside 'races e g at sandown'.
'segats' in reverse letter order is 'STAGES'.
(Other definitions for stages that I've seen before include "Phases" , "Stations" , "Raised platforms" , "Positions or points in a process" , "Platforms on which plays are performed" .)