The policeman called for 'backup' [=called for other police to come and help him] because the suspect was carrying a gun.
There was a traffic 'backup' [=('Brit') 'tailback'] for miles on the road because of the accident.
His role on the team is to be a 'backup' to the regular quarterback.
(..) - They were detained when she was . - What happened to her? Juvenile Reclamation Project... ...for five years. - Shit. We're gonna need backup, but keep it minimal. You sure about that, sir? I want a chance to talk to her before she disappears into one of Creedy's black bags. Who's that? (..)2005 V for Vendetta