Mick, there's a couple of things you can do.
1)Nothing-leave the deep like it is cross comb and all and just let them have that box and allow them to move up. You can manage the other boxes.
2)Go in with a serrated knife and cut the cross comb out but leave the feral comb attached between the frames. You can stagger the frames in that deep, one feral, one frame with starter strip, one feral, one with starter strip.
Leave honey on the outside, brood in between.
Can't predict however that they will keep things clean even after you do the repairs but the staggering should help.
3) Removing the feral comb entirely, as M.B. pointed out should not be an option, but if you want to do it as an experiment, or to satisfy your desire to have a cleaner looking hive, just remember they worked hard to draw those 7 frames and you will be bringing them back to square one.