Hello fuzzy brain having done about half the crossword ..I am little puzzled by 21 down I thought it could be Rooting and perhaps 21 across could be Rash
i have the following. I hope they're correct:
15d. Potentate. ten = half a score (in clue); tate = gallery.
18d. Isobaric. This is a type of pressure. I see = 'is'; no hindrance = 'o bar'.
Hope this helps.
Should have said, regarding 'Potentate' that the 1st 2 letters 'PO' also refer to half of a musical score (opus) written upwards, i.e. 'OP' ... if that makes any sense to you.