Bloody hell. Thanks whetstone, and apologies if I've broken convention. I'm new to this, so it's inadvertent - I'll read jono's links with interest.
Regarding the every second letter device, I thought it quite original, only to find it used twice in an old Arachne I'm attempting to solve while on holiday:
Maidens, periodically need bluesy tunes (6)
Roman emperor regularly ignoring advisor, called time on Scotsman (10)
Arachne, of course, had the good grace to place the definition in its proper place!