2636: Happy ever after

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   Extremely sick and unwell, dispersing in Glasgow? (8)

25   Obduracy shown by most of gang outside North London? (8)

30   Zigzags in descent at swimming (9)

31   William (Bill) covers at least two feet (5)

35   European industrial gear grabs earth, perhaps (6)

36   Joyous cry from house, talk-show host belting out piano (6)

37   Character on staff, ultimately retiring, all but split (1,4)

38   Spiders managed to perish, rolling in lava (8)

41   Rancid point, between the lines (6)

42   Snare docked dog crossing lake (8)

Down

2   Elastic in sterile pants (9)

3   Helpless Australian city girl (5)

4   One who designates condition about Stoker (7)

5   Old injury upset Muslim lord (5)

6   Unending pasta sauce initially choked some very bright sparks (6,4)

7   Hard, outwardly adversarial, near the bone? (7)

8   Sequel once changed persuasive speeches (10)

17   Blimey – nerds without clothing (3)

18   In return, he can never hurt (2,8)

19   With urgency, secretive about missile (10)

22   Bird in flight around India, migrating when it’s cold (9)

26   Drifters, better topless, heading south (7)

28   Mostly consider playing lower parts of duet (7)

29   Endorsement, but not the second, gets through (3)

32   Bacterium stage suppresses resistance (5)

33   Metal bogey regularly jams in wheel alignment (3-2)          

Download a printable version here.

A first prize of £30 and two runners-up prizes of £20 for the first correct solutions opened on 29 January.