Put half your money to begin with on red, returning to make a killing (6)
I believe the answer is:
murder
'a killing' is the definition.
(murder is a kind of killing)
'put half your money to begin with on red returning' is the wordplay.
'put' indicates putting letters inside.
'half' indicates taking half.
'to begin with' says to take the initial letters.
'on' means one lot of letters go next to another.
'returning' says the letters should be written backwards.
The first letter of 'money' is 'm'.
'm' after 'red' is 'redm'.
'redm' in reverse letter order is 'mder'.
'your' halved is 'ur'.
'ur' put into 'mder' is 'MURDER'.
'to make' is the link.
(Other definitions for murder that I've seen before include "Kill" , "Deliberate homicide" , "Homicide (very noisy when blue)" , "very difficult task" , "Red Rum (anag)" .)