Black marketeer betting on four (4)
I believe the answer is:
spiv
'black marketeer' is the definition.
(I've seen this before)
'betting on four' is the wordplay.
'betting' becomes 'sp'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'four' becomes 'IV' (Roman numerals for four).
'sp'+'iv'='SPIV'
(Other definitions for spiv that I've seen before include "wheeler-dealer" , "Disreputable and flashy man" , "Flashy black marketeer" , "Agreement" , "shady character" .)