Teacher, 'sittar' - with a double 't' - is listed in Chambers Dictionary, though I personally have never seen it thus spelt. Since the question stated the correct answer had six letters, I simply assumed that Wetherspoon's setter had chosen that version. Simple as that.
Mybutty</b., given the letter-pattern, there is no doubt in my mind but that 'erratum' is correct. Assuming your other letters for 3 down are right, this leaves S - T - R - for the musical instrument. I can find no Indian instrument that fits that pattern. So...are you really sure about the 's' and the 't'?