tall order vs undertaking

tall order and undertaking both are nouns.

Word	Noun	Adjective	Verb	Adverb
tall order	Yes	No	No	No
undertaking	Yes	No	No	No

As nouns, undertaking is a hypernym of tall order; that is, undertaking is a word with a broader meaning than tall order:

Other hypernyms of tall order include labor, labour, project, task.

tall order (noun)	undertaking (noun)
a formidable task or requirement	any piece of work that is undertaken or attempted
	the trade of a funeral director