### Clue: Work on an inlay that's iridescent (7)
### Answer: OPALINE 
 ### Explanation: Definition: iridescent
  OP($$op$$us,a work of music or literature) **placed before**(on, in a down clue) A LINE(a narrow continuous mark cut into a solid surface and then filled by the same or another material of a different colour/texture/etc, the inlay.  Though inlays could also be broadly shaped.)
Defn:  Opaline is the exhibiting of different colours, like in an opal.  Iridescence is the range of colours that you see on soap bubbles and oil films.  I don't think they're the same phenomenon though, scientifically speaking.