fill–in 
/ˈfɪlˌɪn/ noun
plural fill–ins /ˈfɪlˌɪn/ noun [count] : someone who takes the place of another person who is away for a short time : someone who fills in for someone else
▪ She performed well as the emergency fill-in. [=replacement, substitute]
▪ She performed well as the emergency fill-in. [=replacement, substitute]
