The quotation is "Things fall apart; the centre cannot hold", from Yeats's The Second Coming. Thus the centre letters of each answer (3 ex 5, 2 ex 6, 3 ex 7, 4 ex 8, 3 ex 9, 5 ex 11, 4 ex 12) had to be re-arranged before entry into the grid. Appearing as a spiral in an anti-clockwise direction from the central C of the grid is the phrase CENTRIFUGAL FORCE.

Clues Across:

1. (attached) anag. of SIZE+N+ATACHED (ZANTEDESCHIA)

9. (her) FOR + UM

12. (harim) DY in ILL

13. (tamarind) anag. of PEAS + TAMAR (ASPARTAME)

14. (Bodge) rev. of (N)EAPS

16. (swell) ORD in SO

17. (flock) CLAPS + ILL

19. (Ma) (M)AGNATES

20. (Darnley) anag. (YARNED)

26. (Instal) hidden (TAPETI)

28. (prayer) UN rev. in NOSER

29. (pleading) PROLIFER(ATION)

30. (Brainy) SAL + T(ERR)Y

31. (recce) LO + M + A

33. (at) anag. I + LESSONS + A (SESSIONAL)

34. (Stay) EG in ELY

35. (the) S(E)MI + TE

36. (are) ELL + AR in STATORS

Clues Down:

2. (clot) hidden (i.e. similar colours)

3. (Traverses) odd letters

4. (linens) RIP in STED

5. (table) T + L in DEFY

6. (horse) sound of HIDE RANT

7. (alley) A + HERE in DR

8. (Cruddy) FLO(P) + RID

9. (as) OR + S in FAY

10. (tinnies) rev. of PUT in OC LET

11. (Spurn) anag. of CASINO in PRESS (PROSAICNESS)

15. (let goo) CAN + S + T

18. (Chaste) anag. of N MORAL CERTAINTY minus CLARITY (ORNAMENT)

21. (Mughal) anag. [ambiguity of ALGUM/ALMUG resolved by key phrase]

22. (Bora) anag. A + LIFES plus S (FALSIES)

23. (plan) U + P + SCALE

24. (fund) HO + anag. FUN in R and L

25. (snarly) rev. of GNAR in OE ["jokey" used as red herring for "K" of
KEATS]

26. (steel) OR in anag. of STEL

27. (Arnie) anag. of RNIE + OR

31. (sting) (F)LIRT, with L for R

32. (Morse) anag. of MORE