Explanations

The theme of the puzzle was Richard Wagner's The Ring of the Nibelung to mark to 200th anniversary of his birth in 1813. The four parts of the Cycle were referred to in a ring around the outside of the grid.

With regards to the additional hints, the initial letters of clues 13a to 25a gave ALBERICH, the central character of the Ring Cycle. A similar device was intended to be in place for clues 20d to 26d to give RING, but the setter apologises that a couple of careless last minute clue changes somewhat broke the pattern. Hopefully this did not spoil your enjoyment of the puzzle.

Across

1. Part 1 of The Ring Cycle: RHINE (liquid) + GOLD (solid) => RHINEGOLD
2. TEAM (ally) mostly => TEA (sustenance)
3. SI (is on the rebound) + R (first Romance) => SIR (chap)
4. UN (a Parisian) + MEAN (vulgar) + T (piece of Text) => UNMEANT (was in error)
5. IN CAN (is preserved?) => INCAN (civilization)
6. ED (boy) containing (bounds) MEN (boys) => EMEND (better)
7. C (caught) + HE (man) + QU and ER (two queens) => CHEQUER (alternate squares)
8. I (current) + R (right) + ISH (his problem) => IRISH (language)
9. ABLL (ball play) minus L (missing left) + Y (youth) => ABLY (with skill)
10. E (energy) + CLAIR (girl = Clare heard) => ECLAIR (cake)
11. R (right) following HA (laugh) + ME (I) backwards (get back) => HAREM (the ladies)
12. UC (Unusual Cotton seeds) contained in Y (yankee) + CA (state) => YUCCA (it grows)
13. MUSE (Clio) contained (caught) in AD (commercial) => AMUSED (caused to cackle at)
14. H (hot) following GAS (fumes at first) => GASH (tear)
15. N (head of Nosferatu) contained in SACK (case) => SNACK (bite)
16. CUS (copper's) + AL (nearly completely=all) following (positioned by) AC (electricity) => ACCUSAL (charge)
17. OSER (rose entwined) containing I (tip of Iris) => OSIER (twig)
18. TE (mostly X=ten) + R (right) after EN (space) => ENTER (key)
19. AT (in place) + INGLE (hearth) => ATINGLE (very excited)
20. S ('s) + IN (present) => SIN (lapse from the straight and narrow)
21. HOME (house) minus M (Magazine headline missing) => HOE (garden)
22. Part 3 of The Ring Cycle: (Mime's foster) child => SIEGFRIED

Down

1. HUNCH (guess) containing A (answer) => HAUNCH (bum)
2. NUMBER (arithmetic) + CRUNCHING (chewing) => NUMBER CRUNCHING (cryptic reference, &lit)
3. Anagram (re-installed) urged + natural gas => GRANULATED SUGAR (that's sweet)
4. LIT (on) + ERA (time) + RY (railway) => LITERARY (learned)
5. Part 4 of The Ring Cycle: end => TWILIGHT
6. AS (while) + initials (primarily) Caught In Introspection => ASCII (way to interpret characters)
7. HE (chap) contained in SLLAC (around about) => SHELLAC (coat)
8. Part 2 of The Ring Cycle: action => RIDE
9. Initials (primary) Summon, Urge, Entreat => SUE (&lit)
10. Part 2 of The Ring Cycle: selector => VALKYRIE
11. REAL (proper) + IGN (gin usage) => REALIGN (adjust again)
12. MA (mother) + HI (greeting) containing (embracing) A + RAN (shot) => MAHARANI (Indian lady)
13. Contained (nestling) in yoUR Nest => URN (it could keep your loved ones)
14. Contained (crucial element) of FairiSLE EVEningwear => SLEEVE (&lit)
15. A + ITCH (need) => AITCH (omitted by Cockney)
16. Part 4 of The Ring Cycle: group => GODS