19 across (two words) is commonly associated with an octet. Each member of the octet defines one further unclued light.

Across

11 Masseur also kneaded old reptiles (11)
12 Country’s northern frogs, newts etc with no acid measure (7)
13 Tease one brunette, vacuous crone (6)
16 Very great network’s backing Sturgeon (7)
17 Personify old emperor and knight, judge admits (9)
18 Stare at faceless spectre (4)
21 Metal wires from Scot’s very regal people (7)
25 German scientist and short German painter (6)
26 Certain to take oral exam almost last (7)
27 Memo about area with e.g.