What does "leave clear on its left" mean in UK regulations relating to collision avoidance on non-runway landings?
Seems understandable to me.
It may be more clear with commas...
"(a) when landing, a flying machine or glider must leave clear, on its left, any aircraft which has landed, is already landing or is about to take off;"

Basically leave enough room between you and any aircraft which may be on your left as you pass them.

The grammar may be a little dated but still perfectly accurate.