A number backed a good time sport (7)
I believe the answer is:
netball
'sport' is the definition.
(netball is a sport)
'a number backed a good time' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'backed' says the letters should be written backwards.
'a good time' becomes 'ball' (I've seen this in another clue).
'ten' in reverse letter order is 'net'.
'net'+'ball'='NETBALL'
(Other definitions for netball that I've seen before include "Team sport" , "Team game for tall Ben" , "Team game/sport" , "After this, you get to serve again" , "Team game, played mainly by women" .)