A bit of sophisticated semantics, there are several ways of understanding it, just check that Fake something Dictionary:

(1) The letter A is doing a runner because of you, so the A is no longer there but the a serving as an indefinite article in his sentence has stayed on. It does not know you or ignores you, but A has run away.

(2) You will know about doing a runner when you look in the mirror and realize that you have mutated into Yeti, the snowman during the course of time. In this case, you are doing a runner ( not A runner) from yourself.

(3) If you ask if I know about doing ' a runner'. I say no. I only now about doing 'an runner", and that's what I get from that Fake Dictionary.

BTW, are you perhaps one of the co-authors of this fine dictionary?.....hahahaha