WordNet sense 1 (someone who takes the place of another (as when things get dangerous or difficult)):

fill-in, relief, reliever, stand-in, substitute, backup man

WordNet sense 2 (a subordinate musical part; provides background for more important parts):

accompaniment, support, musical accompaniment

WordNet sense 3 ((computer science) a copy of a file or directory on a separate storage device):

computer backup

WordNet sense 4 (the act of providing approval and support):

backing, championship, patronage, championship