step-in

(noun) short underpants for women or children (usually used in the plural). Synonyms: pantie, panty, scanty.

(verb) get involved, so as to alter or hinder an action, or through force or threat of force. Synonyms: intervene, step in, interfere, interpose. Examples: "Why did the u.s. not intervene earlier in ww ii?."

(verb) act as a substitute. Synonyms: substitute, deputize, deputise, step in. Examples: "She stood in for the soprano who suffered from a cold."