Obviously our friend Michael Mepham is not a musician because the answer for 19d does not place middle C on the second line. As a pianist and church organist I can confirm that using the G-clef, middle C is placed on a little ledger line all by itself, between the bass stave and the treble stave. The second line in the G-clef is the note G, not middle C.
I don't know about Encarta but Chambers says "a clef, the treble clef, on the second line of the treble stave,marking G", which seems to prove Porilo's point.
I thought every piano pupil learnt E(very) G(ood) B(oy) D(eserves) F(ood) as a way of remembering which note was on which line !
I have been out all day and just been finishing off a few clues left by my husband. I could not believe that MM had given G CLEF as the answer (with peonage) and scrolled back to find if anyone had queried this. Porilo and kettledrum are perfectly correct; the G-clef places G on the second line. By the way, I learnt Every Good Boy Deserves Favour, but that is a bit old-fashioned!