ACROSS
 1. JAB BERK outside W O C = JABBERWOCK <monster>
 7. "SOME" = SUM <problem>
 9. BRILL ←GI← = BRILLIG <when the 14a 22a>
10. <metal> THORIUM = (H + OR I) inside TUM
11. <real> TANGIBLE = TANGLE outside I B
12. SH Y outside LIT = SLITHY <description of 18a>
14. <those who 22a> MOME RATHS = ←RE← inside MO MATHS
16. "BAYS" = BAIZE <green play-area>
18. <creatures in the 24d> TOVES = V inside TOES
19. (ROE + ADVERT)* = OVERRATED <worse than claimed>
20. <overlook> FORGET = FORGE T
22. <what 14a did> OUTGRABE = OUT GARB* + _E
25. (.p)RICOT TA(r.) = RICOTTA <dairy product>
26. <reveals> UNVEILS = VENUS* outside IL
27. D out of DYES = YES <of course>
28. <went back to war> RE-ENLISTED = DESERT* outside NILE*

DOWN
 2. <outlandish> ALIEN = LIE inside AN
 3. <game> BILLIARDS = (BIRDS ALL)* outside I
 4. RELATION outside GU = REGULATION <management>
 5. OAT + H = OATH <curse>
 6. K_N_ O_ L_L_ = KNOLL <hilly area>
 7. (THIS IS NOT)* = SHINTOIST <adherent of Eastern faith>
 8. <how 16d were> MIMSY = (.o)M I'M SY(n.)
 9. BA (M_ inside THAT) = BATH MAT <washroom feature>
13. <typical> AS PER USUAL = ASP ←(L + AU SURE)←
15. ICK inside M_ AVERS = MAVERICKS <rebels>
16. <there were all 8d> BOROGOVES = BORES outside (_O_ + GOV)
17. <constant> ENDLESS = D _L inside E NESS
20. <journey> FORAY = FOR A Y
21. ENTERPRISE - PRISE = ENTER <input>
23. (A MINE) = AMINE <organic chemical>
24. W + ABE = WABE <place for the 12a>