Apparently this puzzle has been set by a compiler named Hamish who uses the pseudonym 'Soup'; it's his first for The Guardian.
He has posted this to The Guardian's Monday cryptic crossword comments:
SetterSoup 4 Jan 2016 08:26
Hello,
I've set the Genius puzzle this month - my first for the Guardian - but sadly the gremlins have crept in and a) the form's missing and b) so are two clues. These are:
18d Slide down a mountain in a bearskin (3)
26d Girl's tummy's beginning to be fat (4)
I can't do anything about the form, though; I've emailed the Editor...
Cheers,
Hamish (Soup)