Crack April's crossword competition to win The Elements of Eloquence: How to Turn the Perfect English Phrase, by Mark Forsyth.

The subsidiary indications, or "wordplay", in the clues will lead to the answers plus, in each case, one superfluous letter. These letters specify what is to be highlighted in the completed grid, a total of 21 cells in a number of groups. 

Interest in posture taken by principal of University (6)

Across

1.  Interest in posture taken by principal of University (6) 

5. Scholars made off when in grip of pain (7) 

12.  Nymphs involved in sin erred (7)

15. Boyfriend seen around European office (6) 

16. Gratuity given to guide, one regularly napping  (7)  

17.  Paramour lurking in shallow veranda (5) 

18. Fury in Hades, in more than one manner of speaking (8) 

20. Lachrymose Arab enthralled by performing fluter (7) 

21. Cursed uninhibitedly, nursing fifty sores (6) 

28. Avoid being seen holding form of contraceptive (6) 

30. Car with clever text device (7) 

32. Extend thanks to Rector involved in professional play (8) 

34. Land given back by meek Belgian (5) 

38. Approaching a river, riding in form of engine (7) 

39. Blue nut processed in small vessel (6) 

40. Needing oxygen, face ground in which bird mostly nests (7) 

41. Deceive ladies involved in motivational research (7) 

42. Most surreptitious position in schedule secures agreement (6) 

 

Down

1.  Disconnects security device, making son almost tumble? (7) 

2. One who's just landed painting ahead of competitor (7) 

3. Someone who wants attention in north- eastern region, initially (6) 

4. Espies first of officers, giving informal greeting (4) 

5. Drink produced by a philosopher, mostly (4) 

6. Friend from North America concluding "XX" (5) 

7. A legislator abundantly supplied, with a vessel holding oil (7) 

8. Take apart codes - it's intriguing (7) 

9. Application not needed by one learning to draw (6) 

10. Top musician forced way to rostrum, refusing to play (7) 

11. Sees snags about German exit (6) 

13. Coin, one circulating around old city (4) 

14. Theologian chewing gum with little hesitation - a stupid person (3-3) 

19.  Some familial characteristic associated with a bone (5) 

22. National church abandoned by Charles II, one seen as "merry" (7) 

23.  Roofing worker, one left to make sideways move (7) 

24. Lift has one careering through Peru, unexpectedly (7) 

25. Nod from obsessive person ending with arrangement to meet (6) 

26. Rogue buccaneer dropping article right beside one - one thing added to drink? (3,4) 

27.  Again choose elements out of pure eclecticism (7) 

28. Poet muddled up Shakespearean camp follower's name (6) 

29. Firm supports offered by rock-like substances? (6) 

31. Former pupil spilled blood on beginners in year one (3,3) 

33. Greek islander initially removed inner membranes (5) 

35. False god, one found in eastern island (4) 

36. Oath, say, taken by a doctor (4) 

37. Big or small knot in wool (4) 

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