Informer ( Slang ) (5)
(Other definitions for grass that I've seen before include "Pasturage" , "Eg, bent or fescue" , "Give away" , "one may be urged to keep off" , "Lawn plant" .)
(Other definitions for nark that I've seen before include "Annoy (colloq.); informer" , "Grass; irritate" , "Cause minor annoyance" , "Annoy; police informer" , "Annoy - informant" .)