Put the ___ on (stop) (6)
I believe the answer is:
kibosh
(Other definitions for kibosh that I've seen before include "Hibs O.K. (anag) - put a stop to" , "Finish off" , "Put the ___ on - rubbish" , "In slang, put the . . . . . . on - end it" , "Fatal to put this on" .)