I've spoken English all my life but the sentence Is “there” a postpositive attribute or an apposition of “on the wall”? is French to me.
Hell, let's break down the sentence and build it:
There is a clock.
To say 'there is', as we know, only means that in the space you've defined, you may find a clock. So, let's place the clock:
There is a clock on the wall.
This narrows down the possible locations of the clock, but, without context, this wall could be anywhere. Because we're currently talking about a candy shop, I'll assume the sentence means:
There is a clock on the wall in the candy shop.
English has a nice way of avoiding the need to mention a place twice. A candy shop is a 'place' and a place is later referred to as 'here' or 'there'.
There is a clock on the wall there.
I haven't the foggiest idea how this relates to your question, but I'm hoping this'll clear it up.