I've got a full grid with the message around the edges and that spelled out by the extra letters, but before I move on to the next stage there are two I can't fully parse....
5d Bone of the ear bears mark associated with diffusion of fluid (7)
I can see a 4 letter "bone of the ear" and M for "mark"? but that leaves 2 letters which I can't match to the rest of the clue.
16a Worried about exercise? Hardly! (6, 2 words)
This has an extra letter which I think is "U" but if "worried" is the first three letters then that leaves ASE + U for the rest - where have I gone wrong?
Thanks.