second string

another string to one's bow

Another skill, ability or resource to be used in case of failure.

apron string

A symbol of the domestic ties binding a male to a female (as a husband to a wife or a son to a mother).

harp on one string

To dwell on a single subject with disagreeable or wearisome persistence.

harp on the same string

To repeatedly say the same thing.

have second thoughts

To change one's opinion, or be uneasy about a previous decision.

just a second

A short period of time, typically anywhere from a few seconds to several minutes or more.

play second fiddle

To play a subsidiary or subordinate role to someone or something else.

second banana

A comedian who plays a secondary or supporting role, especially as straight man and traditionally in vaudeville or burlesque theatre.