second string

Sports. the squad of players available either individually or as a team to replace or relieve those who start a game.

a secondary position, status, or group, as in a level of corporate management.

Origin of second string

1635–45 in sense “backup, resort,” alluding to a second bowstring; 1860–65, applied to racehorses; 1950–55, to sports teams

sec·ond-string, adjective sec·ond-string·er, noun

True, she was only a second-string nominee, after Tom Daschle had to bow out because of those tax problems.

Will Kathleen Sebelius Win in the End? Legacy Tied to Obamacare’s Outcome |Michael Tomasky |April 11, 2014 |DAILY BEAST
But for all the advantages Assad has handed to them, they cannot dislodge his second-string units from the city.

Syria: Would a No-Fly Zone Help the Rebels Oust Assad? |Barak Barfi |September 11, 2012 |DAILY BEAST
Nor was it the second-string outfit of the Moguls that would come.

Local Color |Irvin S. Cobb
Any second-string fellow that looks as if he was being held back?

Quarter-Back Bates |Ralph Henry Barbour

Driscolls firm for starting with a second-string line, but I dont like it.

Full-Back Foster |Ralph Henry Barbour
He took me from a second-string team and gave me a chance to make good on the Giants.

Baseball Joe Around the World |Lester Chadwick
I suppose Driscoll wants to bring him around for second-string man.

Center Rush Rowland |Ralph Henry Barbour

second string

mainly British an alternative course of action, etc, intended to come into use should the first fail (esp in the phrase a second string to one's bow)

a substitute or reserve player or team

sport

second-rate or inferior