Green’s Dictionary of Slang

1. an Italian living in London, orig. spec. in Clerkenwell [? image of Italians as anarchistic, i.e. SE radical].

1938		F.D. Sharpe Sharpe of the Flying Squad 208: Then one asked: ‘Are there any ‘Raddies’ here?’ (‘Raddies’ means Italians).
1977		D. Powis Signs of Crime 198: Raddie An Italian, particularly an Anglo-Italian domiciled in London. Originally a term descriptive of Italian families in Clerkenwell.
1981		(con. c.1900s) A. Harding in Samuel East End Und. 112: Long Hymie did it, and some of them who went with the Raddies, the Italian mob.

2. a radical.

1964

E. Parr Grafters All n.p.: He discovered that his acquaintance was a bit of a raddie at heart [DSUE].