have-a-go hero

noun1

1. •
   (UK, informal) An ordinary member of the public who intervenes, often at personal risk, to stop a troublemaker, hooligan or ne'er-do-well in the act of committing a crime.

   • Jack Straw today admitted that he was "always uneasy" about the government's refusal to offer greater protection for have-a-go-heroes as he explained plans to review the laws on self-defence. — 2007