sorry if I haven't explained it very well.
Some of the down answers have a blank square at the top when the grid is filled - so LED (def 'under control') starts at the second square of 7d.
When you've filled in all of the entries you'll see that there are only four letters entered in the top row - when you add the 'newly arrived group' (9 letters) you must avoid creating any new words in down entries. You'll then see there are four blank squares left, and when you put the 'weapon' (in normal order) into these four squares you'll make new words for the four remaining entries.
I hope this helps if you go back to the puzzle, brendan.