The third letters of extra words in two dozen clues spell out a 35 (in ODQ 7&8), associated with the 19 31, which explains the remaining unclued lights (all different real words), one of which is the cause of the others and must be shaded.
 

Across

 
4    Packed out balls getting odd crew dancing (11)
11    Italian aboard railway takes time travelling (9)
12    Ample jumper one’s wearing (5)
13    Plant gardener’s wife put in ground (grassy) (11)
14    Public showing animation as stunner stripped (4)
15    Jack married after secretary insisted (3)
18    Tom Boon designed buildings for the public good (7, two words)
22    Solemn type wearing...