If the answer is NIKAB, then KA is in IB (Latin abbreviation meaning "the same place". The N at the beginning may be short for "in", but not sure about that.
I'm quite confused about 6 down. I have NIKAD (the N is the letter omitted in word play) KA as mentioned already, but ID means SAME, not 'in the same place' which is IB (abbrev.) as also mentioned above. There is also a word NIQAB for a veil - not possible because of the Q.
I think I,d go along with nikab, seems to fit with Chambers. Crossword now complete with twelve letters highlighted.(a tough puzzle). Ive got the aphorism though I am not sure of its significance or origin. Any subtle thoughts?
I've found what I think is the aphorism and I've completed the grid apart from 9d, where the only word I can find that fits (GRINDSTONE) doesn't seem to bear any relation to the clue. Can someone please tell me where I've gone wrong - or explain it if it's right please? Once I've stopped going mad I can start work on the conclusion!