Question Author
Thanks Professor. This will give me something to think about this evening.
For those respondents asking for the clue this is one of the lights for which no clue is given other than the overall theme. In this case the clue to the theme is that each unclued light can be preceded by the same word, and a clue to that word is 'down'. It becomes apparent after partial completion of the grid that the common word is 'blue'.