This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. having an inadequate number of workers or assistants

adj. inadequate in number of workers or assistants etc.

"# of stores that could have been considered "understaffed" - 0 out of 8"

inadequate    unequal   

short-staffed    short-handed    undermanned   

undermanned    undernourished    short-handed    under-resourced    underdressed