From Longman Business Dictionary
understaffedun·der·staffed /ˌʌndəˈstɑːft◂ˌʌndərˈstæft◂/ adjectiveadjHUMAN RESOURCES a company, organization etc that is understaffed has fewer people working for it than it really needsThe factory inspectorate is seriously understaffed at the moment.
→ opposite overstaffed