In ancient Greek, all present indicative forms of the verb "to be" (except second person singular) were enclitic, and so followed the normal rules for enclitic accent, e.g. ἄνθρωπός ἐστιν, but φίλος ἐστίν. (Oikonomos 19-20.) In the preceding examples, ἄνθρωπος develops a new final accent to accommodate the enclitic; φίλος, however, would have to have two consecutive acute accents (which is impossible), so the enclitic itself develops an accent.
At the beginning of sentences expressing definitions or "there is/are," ἐστιν receives an accent on the first syllable, e.g. Ἔστι χρυσοῦ κτῆμα τιμιώτερον.
Μοnotonic belongs to modern Greek. If you want to use the ancient word ἐστιν in a modern sentence, you still follow the rules for accenting (or not accenting) enclitics, which apply in modern Greek as well.