Green’s Dictionary of Slang

an informer.

1959		F. Norman Fings II i: Nuffing but a pair of grassers.
1973		‘P. Alding’ Field of Fire 50: It must’ve been a grasser [...] Informers were the biggest peril.
2005		B. Hare Urban Grimshaw 78: Inside there were rules, and rule number one was: ‘No Grassers.’.