I have just started this crossword so you are more advanced than I am!
I think the last one is a plant but I can't totally parse it
I think that the extra letter is the F in off
Start of Job= J
then a 2 letter word for "when"
Boxes = goes round, "in"
but the other 2 letters something to do with "men supply"?
If the surplus letter was the "N" and it was a compiler's errorspelling off rather than of, then I could parse it.....but my answer may be totally wrong anyway - but it does agree with your letters!