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.
(..) Got that? Fuck you. He killed one of the hostages. We move. Mac, Blain - the nest. Billy, Poncho - the guard. Hawkins, Dillon - backup. As soon as they're set, I hit the fuel dump. One down. What the hell's he doin'? What the fuck...? Showtime, kid. (..)1987 Predator
(..) evaluate the situation and take charge. Rendezvous points and radio freqs are indicated and fixed. - AWACS contact on four-hour intervals. - Who's our backup? No such thing. This is a one-way ticket. Once we cross that border, we're on our own. This is getting better by the minute. Billy! (..)1987 Predator