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.
(..) - We are... - I've been looking for you everywhere. Where the hell have you been? - You know these two? - Of course I know them. They are Xania's backup dancers. They go on in the third act... ...with the Komodo dragon and the snow machine. Why do you think they are dressed like that, for fun? (..)2006 The Pink Panther