Twelve unclued lights may be grouped into three quartets and their unchecked letters spell out ask engineering-academy.gov.uk; each quartet comprises two natural pairs. All unclued entries are normal words, one is in Merriam–Webster and one is thematically linked to its quartet via a Broadway musical.

Across

1   Maybe PC speed unit cut after unexpectedly taking van (8)

6   About to display ancient nut (6)

12   Local scolding somehow raises your back (8)

13   Developer of global movement almost left (5)

14   Dogged has-been hooligan welcomes trouble (6)

15   Chain two blokes side by side (6)

20   Returning from Dubai, unnerving bore (5)

21   American people embracing setter’s vigour (9)

24  ...