Hello, Elle!
Thank you for the very complete explanations!
I think, on reflection, I may have heard of "gazumping".
Could it be linked to "gazundering", I wonder?
Now today's puzzle!
I did not want to scare you off by saying it was a toughie....but I found it to be so!
I got 1a straight off, but the parse took a bit of thinking about.
The last two letters in the answer is the name of a mountain in Karakorams, better known as Masherbrum.
Munro is the definition, but not a mountain.
18 is a Czech river which I had not heard of (and probably a few like me!)
12a is an old play that I had not heard of before, by John Milton.
19d is the name of a London theatre.
Hope these hints are a help, Elle.
Just ask if you need any more.