Any of a wide range of things seen as exceptionally good, excellent or cool:
“The baw landit at his feet an’ he hit it a belter.”
“Who was that wee belter ye were chattin’ up there?”
“Yer new motorbike’s a belter!”
“Ah landed tickets tae the concert fur cheap! Ya pure belter!”
See last week’s (posted late) edition, it’s a pure belter.
Thanks to a new reader, Laura K. for the suggestion!