The unclued Across lights are of a kind, as are the related unclued Down lights. The yellow and red squares, when read in order row by row, confirm the theme.

        Across

   1    Feel rip-off by stewards providing obligatory features on board (4,10)

11    Section of grammar review of Aeneid (three chapters) (9)

17    Pulling evenly, stripped part of a bone (5)

20    Vintage model – it crashed (3-4)

21    Daub top layers of special paint on gatehouse (7)

24    Threaten to drive around small state (7)

25    Learn about description of organ (5)

26    Passions and fashions! (5)

28    Small middle-distance runners are happy folk (7)

31    Go...