Mark slinging every other character out (4)
I believe the answer is:
sign
'mark' is the definition.
(I know that mark can be written as sign)
'slinging every other character out' is the wordplay.
'every other character out' indicates alternate letters.
The alternate letters of 'slinging' are 'SIGN'.
(Other definitions for sign that I've seen before include "Indication - put one's name down" , "Notice; omen" , "Augury" , "Indication; symbol" , "Public notice; gesture" .)