12a. Nasty weapon (5) M?L?Y
(I have MALKY as a term for being slashed, chibbed (or headbutted), so assuming a razor might be a MALKY?)
20d. Twelve inches (3) ???
(I have TAE as a Scottish term for a foot is TAE apparently).
24a. Go. A row in knitting (4) ????
(I have GANG as I believe its a term for leaving, and a Shetland knitting term for a row in knitting).
Wiki gives - "Malky/Malkie, slang term originating from Glasgow and the West coast of Scotland, used to describe slashing someone with an open razor. Comes from the rhyming slang Malcolm (Malky) Fraser (Razor)"