a submarine.

a substitute.

a submarine sandwich. hero sandwich.

a subcontractor.

a sublieutenant.

a subordinate.

a subaltern.

British . an advance against one's wages, esp. one granted as a subsistence allowance.

Photography . a substratum.

to act as a substitute for another.

Photography . to coat (a film or plate) with a substratum.

noun

alternate , replacement , stand-in , surrogate , auxiliary , backup , below , second , standby , submarine , u-boat , under , understudy

verb

fill in , stand in , supply