Each of ten clues contains a misprinted letter in the definition part. Corrections of misprints spell a word which can be read as an indicator of the unclued lights in each of four columns of the grid.
 

Across

 
1 Revolutionary avoids accepting one god (7)
6 Destroyed each bit of opium (7)
11 Priest and paramour lapsing? (6)
12 Tense champion catching alien in addition (7)
14 Mouldings attached to front of impressive gateway (5)
15 Dredge section mostly exposed (5)
16 See current process releasing a vitamin (6)
17 Rascal having problem carrying Albanian coin around (6)
19 Worn-out horse, mad at vet, I smarten up (9)
21 Chief among...