It's a very big march this.

[Uncanny]

What role does "this" play in "It's a very big march this."?

 

[emsr2d2]

It's a different (and informal) way of saying "This is a very big march". It puts the emphasis on "this" so it can be compared with other marches.

 

[Amigos4]

The sentence would not be natural in AmE. As emsr2d2 points out, “This is a very big march" is more natural.

 

[Rover_KE]

It's a common enough informal construction, especially with a comma before 'this'.

In a related context, (when the subject has already been mentioned) a comma is mandatory;

He was a genius, Einstein.
He's a very rich man, Elon Musk.