It's not a sentence, it's a nominal phrase (that's why it doesn't have a capital letter).
"This/is is business" would be «Э́то би́знес». «Э́то де́ло», when used as a sentence, has an idiomatic meaning 'it is a bargain, I agree to this', so it's unlikely to be used to mean 'This/it is business'.
Это is a not just a free-standing pronoun, but also a neuter-gender form of э́тот. So, «это дело» can mean both 'it/this is [a] business' and 'this business'.
Oxford dictionary gives very many meanings, and these should all be included. There is no context given here, with a two word phrase, that would exclude any other meaning. Also, the Oxford dictionary gives synonyms including работа, and занятие. The various meanings are business, affair, deal, cause, occupation, concern, matter, point (as in point of honor), fact, deed, act, case. There are other meanings, but that case and work are refused is something with which I would take issue.
On Wiktionary, it says the word is cognate with English 'deed'. https://en.wiktionary.org/wiki/%D0%B4%D0%B5%D0%BB%D0%BE English 'deal' is cognate with German 'teilen', "to divide". It seems the resemblance is accidental.