1. (verb)stand-in, substitute, relief, reliever, backup, backup man, fill-in someone who takes the place of another (as when things get dangerous or difficult) "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"
4. (verb)substitute, sub, stand in, fill in be a substitute "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet"
5. (verb)complete, fill out, fill in, make out write all the required information onto a form "fill out this questionnaire, please!"; "make out a form"
Definitions of fill-in 
