Green’s Dictionary of Slang

build n.

the cut or style of one’s clothes.

[UK]‘Cuthbert Bede’ Adventures of Mr Verdant Green (1982) I Ch. x: If he foreswore the primitive garments that his country-tailor had condemned him to wear, and adapted the build of his dress to the peculiar requirements of university fashion.
[UK]Hotten Dict. of Modern Sl. etc. (2nd edn) 106: build applied in fashionable slang to the make or style of dress, &c. ‘It’s a tidy BUILD, who made it?’.
[UK]Sl. Dict.
[UK]Farmer & Henley Sl. and Its Analogues.