I also made a mistake with 1a that baffled me at first. Does the answer you're thinking of fit with 3d and 4d? If no, but it does with 1,5 and 6 down, you might have made the same mistake as me.
This is the first of the clues with an extra letter.
Once you've sorted that out, see my earlier post replying to Lavengro, about "what these letters do".
The 4 letter word from the missing letters in the down clues was not so much help, I found.