LOI and I'm completely stumped!
18D: Mobile? Its casing is sound (7)
P?O?E?E
I'm pretty sure all the letters are correct.
I expect I'll need an explanation, too, please!
It's all right, shooty, by that time I'd followed what you were doing and realised what you'd meant to put!
I would never have worked out the answer - I found today's puzzle very difficult.