Gambling on promotion (7)
I believe the answer is:
backing
I believe this clue is a double definition.
'gambling' is the first definition.
(I've seen this before)
'promotion' is the second definition.
(I've seen this before)
'on' is the link.
(Other definitions for backing that I've seen before include "promotion" , "funds?" , "gambling" , "change of wind" , "Endorsement" .)