Thank you buddy and Jono. Leg = on, that's a cricket thing, isn't it? Not a fan of cricket references in cryptic crosswords, and Mrs Strangely Brown won't do cryptic crosswords on account of them.
34d Mayhem:United supporters up for it?
It would appear the answer is SNAFU. Google tells me it’s US slang, derived from a military expression. Obscure or what? Or is it just me?!