The answer is CHEMICAL COMPOUND
The corresponding square is house puzzle
   by Roger Barkan

Common Bonds 
by Roger Barkan

In this cryptic crossword, 8 of the clue are actually two parts and cross over a boundary line.

1. o(p)ec(R)
4+8. de(sic)ant
11+12. me(tho*)dic
13. elm+er
14+15. g(uat+emal)(R)a
16. may+o
17. at sea*
20+21. c(h)ampion
24+25. mo(ti(R))oned
26. c(h)oke
28. espn*
29+31. eva(cu+ate)d(R)
33+34. c(as+sand)ra[zed]
35. e(z)ra
36. arch(2)
37+38. st(eel+d)rum

1. om+egas(R)
2. per+u
3. chute(H)
4. doze(H)
5. edam(R)
6. i+celand*
7. ceram*+ic[e]
8. c+lamps
9. new york*
10. tr(y)on
18. two-star*
19. amo(eba(R))s
20. con+vent
22. finish(H)
23. bedlam*
24. me+cc+a
27. ha+zed
30. core(H)
31. ur(a)l
32. ecru*

DESIC|CANT
 METH|ODIC
GUATE|MALA
 CHAM|PION
 MOTI|ONED
 EVAC|UATED
CASSA|NDRA
STEEL|DRUM

Reading down the letters on either side of the dividers that split the
clues reads, CHEMICAL COMPOUND.  Thus the title, common bonds.