Betting journalist rushed (4)
I believe the answer is:
sped
'rushed' is the definition.
('speed' can be a synonym of 'rush')
'betting journalist' is the wordplay.
'betting' becomes 'sp'.
'journalist' becomes 'ed' (short for editor).
'sp'+'ed'='SPED'
(Other definitions for sped that I've seen before include "Rushed" , "got a move on" , "Moved fast" , "Moved quickly" , "Dashed" .)