The unclued lights (one of two words) and four others clued without their thematic definition are of a kind.

Across

1    Two little boys tapped their heads (6)
7    Birettas seldom reveal this tuft (6)
13    US challenge to foil and dismiss at the wicket (5)
14    Material from The Spectator, London (5)
17    Salary and gratuity, collected by mail (7)
18    Incandescent, he’d be forced to put rubbish out (3,3)
19    Infection covered by the oculist? Yes! (4)
22    C19th biologist’s not left film director (6)
25    Violinist’s bow? No, the other end! (5)
26    Followed the progress, we hear, of Lenten psalm (5)
33    Study Latin on express (4)
36    Live...