Half of Mark's gamble (4)
I believe the answer is:
risk
'gamble' is the definition.
(I know that gamble can be written as risk)
'half of mark's' is the wordplay.
'half of' indicates taking half.
'mark' becomes 'asterisk' (asterisking is a kind of marking).
'asterisk' cut in half is 'RISK'.
(Other definitions for risk that I've seen before include "The possibility of danger - chance it" , "Danger or hazard" , "Possibility of bad outcome" , "Take a chance" , "Expose to danger" .)