The unclued lights (one hyphened and another a novel which is listed in Chambers Crossword Dictionary) display a similar feature — a few of the clued solutions do so too, though to a lesser extent. Elsewhere, ignore an apostrophe.
 

Across

1    Slim, young, lithe person – hot! Just tops! (5)
6    Hosting is problematic as soon as possible (7, two words)
11    Boastful coward showing Cliff a sponge (10)
15    Accumulate stakes (4)
17    Letter about smell of savoury snacks (7)
18    Some shirker twisted marginal reading (3)
19    Dictator books shrubs for TVs (10, two words)
21    Performance by dancing...