I think I spell it as you do Chris. At first I thought 24 ac was a themer, with tbe " big man on board" being COX, and wondered what to do with the O and why I had 16 themed answers.
I think 4 dn is just indicating SI as the chemical symbol for the solution.
I agree 4d is "Si", it's just that whenever I've seen "essential" in a clue before it's invariably referred to the central or core part of a word. In this instance though, they're the 3rd and 4th letters of a 7 letter word, which doesn't seem to tally with the essential or core part of the word - maybe I'm just overthinking it.
I, too, would take 'essential' as referring to the central letter(s) of a word, in this case, I.
Otherwise, 'a 7 letter element in MUSICAL', could also be Sulphur or Calcium, apart from the crossers.