I knew the word “lysis” but only in the first meaning in Chambers, breaking down of a cell, and had to look it up to find its other meanings.

I agree with PB @ 6 that “alto” is more usually used for a female voice these days, and the term “male alto” is used to specify otherwise. The make-up of a choir is usually described on a music score as SATB for sopranos, altos, tenors and basses, with the first two usually the female part of the choir.

Minor quibble with 4dn: From the construction we have to join AIR to ALARM for the reversal before removing the R from ALARM. This means that “ignoring R” has to be used to remove one of two Rs. I see this as a small, and readily forgiveable, imperfection in the clue.

Minor quibble with the blog at 20ac: While I can accept that originally the word ALTO was restricted to a male voice, this is no longer the case. Chambers 2008 gives “contralto, the lowest female voice” among the meanings of alto¹.

I don’t know if anyone’s bothered, but clues from each offer an interesting comparison:

Geographical feature that’s identical with halves switched (MESA)

Nothing but inversion of French city (ONLY).

As you may have guessed I’m on the hunt for an indicator (or two) that will provide the elusive switching operation, and today along they come like buses.

BOBBING took me a little while before the penny dropped, too. Oddly enough it stirred a memory from many years ago (in pre-Indy days) of a puzzle in either the Torygraph or the Grauniad where it was clued similarly, except that the reference was spelt out as “Mr Hope and Mr Crosby”.

BACK END is an expression I’ve only come across in the Midlands. Often as an adjective, “back-endish”, referring to a late summer day when an early nip in the air reminds one that autumn is almost here. Not exactly transferrable to Phi’s current antipodean domicile.

Your explanation of LACONIC is much better than my feeble atempts – thanks.

On LACONIC, I think it’s L + AC + ON IC(E).

Very enjoyable puzzle. Thanks, Phi.