41, 1A, 10 (seven words in total) is a description of a phenomenon. Remaining unclued lights, including one of two words and three of three words, either singly, paired or as a trio, form four examples of this phenomenon. All are confirmed in the ODQ (8th ed.)

Across

11 Leaf parts of liana ice destroyed (8)
12 Agreed drug hidden in grass must return (4)
13 Fish, internally cold and blue (4)
14 Split lip — scare for breathing orifice (8)
16 African statesman’s vengeance on criminals (6)
18 It’s complete in French flag (6)
20 With hi-hat, Mary’s beaten uneven beat (9)
21 In trouble, on...