The theme word is 29 down, which defines each of the other unclued lights (including one of two words) in one way or another. All unclued lights are entered in a thematically appropriate manner.
 

Across

1    A posh French novelist covers good French region (8)
6    Divine sun? Relax, it’s most excellent (6)
10    Made sensual dancers lie with an eccentric (12)
12    I forgot to say, mouths itch (5)
13    Running up a tree to get clean (7)
15    Antelope, or a dozen before noon (6)
16    Captain and sailor suppressing laugh (4)
17    Passes comment without pens (8)
20    Raises tax in end of offshore shelters (8)
21    They...