Thanks - I can see how the final two letters of the answer+one other can mean "highest note" but don't understand how the "missing" or "from" works in the wordplay. Re 4d, the letter that has to be replaced is the middle letter of the fifth word - if you replace it with o, that yields an abbreviation that has to be removed from the plural corresponding to the first two words of the clue giving the answer defined by the last word in the clue (used as a verb)!