Solvers! Thanks for trying my puzzle, and I hope you enjoyed it. Please leave a comment to let me know... Also, if you felt there was room for improvement, don't be afraid to say so (it's the only way I'll get any better, after all.)
I don't think there's anything shockingly difficult in amongst this lot, although a couple of bits of wordplay may be obscure.
ACROSS
1A. anag. TOOFFENDMICE.
9A. gongs (medals) = OBES (i.e. order of the british empire) + eastern origin = E.
10A. Double def.
11A. Double def.
12A. Consider = MULL + alien visitor = ET.
13A. reversal of prima donna = DIVA.
14A. about = RE + smaller crosswords = MINIS + initial letters from "cryptics excluded."
18A. anag. WHATOLDEN.
19A. carried by = ON + tuber and his ilk = US.
21A. working = ON contains anag. BAIT.
23A. anag. TIES + buddy = MATE.
25A. anag. RIOTCEASE.
26A. double def.
27A. some trouble gotten into = HOT WATER + armoured vehicle = TANK.
DOWN
2D. Frank = OPEN contains relation = RATIO.
3D. initials of "intense cold evidently a glacial era".
4D. undisguised = OVERT + hot = H + dispute = ROW.
5D. double def.
6D. '70s band = ELO (i.e. Electric Light Orchestra) + queen = Q + anag. TUNE.
7D. last letter of "student" + protest = RAIL.
8D. take fewer naps = REST LESS + stretch = LEG + last letter of "stress".
9D. double def.
15D. express = MILK + upset = SHAKE.
16D. anag. TENSIONS contains A.
17D. first letter of "squabble" + Neptune's spear = TRIDENT.
20D. anag. GIFTED.
22D. honking sound = TOOT + first letter of "horn".
24D. Pennsylvania = PA contains IS.