Side by Side by Apex

 Side by Side by Apex

The 40 letters of the perimeter (7 words, commencing at 1 across, and reading clockwise) and the 5 unclued lights are connected by a theme which has to be discovered.  Chambers Twentieth Century Dictionary 1972 Edition is recommended.

 ACROSS

 1.  (40) clockwise
 11. Bird producing the highest note with difficulty (6)
 12. A division of boundaries to flutter around one (5)
 13. Excellent excerpt of composer — a revelation (4)
 14. (8)
 15. A youngster's farewell to touch one (4)
 16. Man beginning to look for a goddess (3)
 17. One probably hums the opening by the sound of it (4)
 18. One leading girl astray (5)
 20. House back in place makes something of yarn (6)
 21. A maid who was bound to be seen dancing (4)
 22. Miscellaneous pieces following casual opening of show (4)
 24. Some wax and leap all over the place (4)
 25. A student of classical literature to hoax one and it's funny (8)
 28 Sounds ill-natured but, although fruity, it's not a raspberry (4)
 30. Again comes in with enthusiasm (4)
 33. (7)
 34. Reckon it could be the one you like best (4)
 35. The girl grabbing ring is a digger (4)
 36. Some trouper men give a wave to (4)
 37. Becoming the rage? That's about right (3)
 38. A big one, perhaps, for the ending? (3)
 40. Unusual musical to note must not put out Greek dialect (6)
 41. (6)

 DOWN

 1. A beat mostly coming from one of the wings (3)
 2. (6)
 3. To practise holding an E with variations and discomfort (6)
 4. Some Japanese melody strewn with devices? (4)
 5. Most members of cast had a teasing desire (6)
 6.  ________ and Ale for the masses? (5)
 7. A heartless pretty lady from Poplar (5)
 8.The most original of narrators in a dashing fellow from Terra Firma (4)
 9. To ring out one dozen tunes — no duets omitted (6)
 10. Opening of next tune for people with peculiar marriage system (4)
 19. (8)
 23. Letters for making one of the little things caught (7)
 24. Seem to be up top, two redheads initially performing (7)
 26. Nothing in the American's verse that’s short-lived (6)
 27. He is excitedly beginning to kiss and hug the girl's young man (6)
 29. A well-known green for ending of frustration in girl (6)
 31. Lots of secluded females here hasten to get married (5)
 32. An old hand — one electrifying shows (5)
 33. An abundance of Scotch at any time after show's opening (5)
 39. Trio finally attacking a pastoral song (3)

This puzzle appeared in the magazine Games and Puzzles, 19 February 1977 with an article by Apex on Stephen Sondheim.