Have a look at old music at the end — that's your definition, pointing to Gregorian chant, that ancient style of liturgical singing. For the wordplay, take just the first letter of "Grim" (that's "from the start" doing its job), giving you G, then bolt on an anagram — "messing up" tells you to jumble the letters — of "hearing cantor", which shuffles round to REGORIANCHANT. Stick the G on the front and you've got the whole thing.