I found some clues here easy and some hard enough to defeat me. Any help to finish the top-right corner appreciated! Enjoyable regardless. Thanks Chalmie.
A mixed theme of sevens and cathedrals.

| Across | ||
| 1 | BASILICA | 7 of almost rotten quartz (8) |
| BAd (rotten, almost) and SILICA (quartz) | ||
| 6 | RECUSE | Withdraw note about American English (6) |
| 9 | MANIOC | Mother gets money back for starch (6) |
| MA (mother) get COIN (money) reversed (back) | ||
| 10 | PUNITIVE | Cruel joke I see breaking restraint (8) |
| PUN (joke) I then V (see) inside TIE (restraint) | ||
| 11 | DEADLY SINS | Swirling sand yields bad 7 (6,4) |
| anagram (swirling) of SAND YIELDS | ||
| 12 | EONS | Times reflected in odd parts of San Jose (4) |
| found reversed (reflected) in SaN jOsE (odd parts of, every other letter) | ||
| 13 | SNEEZE | Suddenly stop Poles hiding French evidence of disease (6) |
| frEEZE (suddenly stop) underneath (hidden by) S and N (poles) | ||
| 15 | RAN A RISK | Adult teacher, back in class, took a chance (3,1,4) |
| A (adult) SIR (teacher) both reversed (back) in RANK (class) | ||
| 18 | UNDERTOW | Less than 2 went round swimming hazard (8) |
| UNDER (less than) and TWO* anagram=wnet round | ||
| 20 | TULIPS | These blooming sheep ate fruit without me! (6) |
| TUPS (sheep) contains (ate) LIme (fruit) missing ME | ||
| 21 | SHOT | Sweden very popular? Go! (4) |
| S (Sweden) and HOT (very popular) | ||
| 23 | CANTERBURY | Easily run and hide 7 (10) |
| CANTER (easily run) and BURY (hide) | ||
| 25 | FLORENCE | Look right through barrier at site of Brunelleschi’s 7 (8) |
| LO (look) R (right) inside (thorugh) FENCE (barrier) | ||
| 26 | AVIATE | Fly worried about going through (6) |
| ATE (worried) contains VIA (going through) | ||
| 27 | PSALMS | Hands round singular songs (6) |
| PALMS (hands) contain S (singular) | ||
| 28 | SLEEPERS | 7 Ephesian martyrs which are found under railway lines (8) |
| double definition – Ephesian Martyrs are known as the Seven Sleepers | ||
| Down | ||
| 2 | AGAMEMNON | Enthusiastic male lead in unknown writer’s tragedy (9) |
| GAME (enthusiastic) Male (leading letter of) in ANON (unknown writer) | ||
| 3 | IVIED | One competed among climbers (5) |
| I (one) VIED (competed) | ||
| 4 | ITCHY FEET | Wish to move long distances over a year (5,4) |
| ITCH (long) FEET (distances, plural of foot) containing (over) Y (a year) | ||
| 5 | APPLIER | He asks for a quiet position with very little responsibility (7) |
| A PP (pianissimo, quiet) LIE (position) with Responsibility (first letter, very little of) – could also be … P (quiet) with PLIE (ballet position)… | ||
| 6 | RINGS | 7 dwarves had weightless violins? (5) |
| 7 | CATHEDRAL | Cat gets rid of fish in four-sided place of worship (9) |
| CAT and tetraHEDRAL (four-sided) missing TETRA (fish) | ||
| 8 | SEVEN | Film Victor watched outside (5) |
| V (victor, phonetic alphabet) inside SEEN (watched) – 1995 film with a rather gruesome plot | ||
| 14 | ELECTORAL | Online reader has a line about votes (9) |
| E (online) LECTOR (reader) has A L (line) | ||
| 16 | NOTRE DAME | Mark swallows last bit of cheddar cheese in 7 (5,4) |
| NOTE (mark) contains (swallows) cheddaR (last bit of) and EDAM (cheese) | ||
| 17 | SUPERSTAR | Senior policeman grasses up George Clooney, for one (9) |
| SUPER (senior policeman) with RATS (grasses, snitches) reversed (up) | ||
| 19 | WONDERS | Would red snow melting be one of these 7? (7) |
| anagram (melting) of RED SNOW – the 7 wonders of the world. I don’t think this quite works. A wonder (singular) would be one of the 7. | ||
| 22 | HILLS | Roman 7 sick in quiet retreat (5) |
| ILL (sick) in SH (quiet) reversed (retreat) – the Seven Hills of Rome | ||
| 23 | CENTS | Naive people missing in no small change abroad (5) |
| innoCENTS (naive people) missing IN NO | ||
| 24 | BLIMP | Airship this person’s used to break British record (5) |
| I’M (this person is) inside (used to break) B (British) LP (record) | ||
*anagram
definitions are underlined
Thanks PeeDee
You are right with your parsing of 6ac, it is RECUSE. I believe 6dn is [st]RINGS (weightless violins) but I was hoping that you were going to explain the connection with “7 dwarves”.
It is the seven rings from the Lord of the Rings. Just figured it out.
PeeDee
Thanks. I had a feeling it might be that but I’m not familiar with Tolkein’s works and the relevant Wikipedia article was of no help (or at least the bits of it that I read).
I have a wife and two children who are LOTR mad and so over the years I have imbibed much more about this subject than I ever really wanted to. There are The One, The Three, The Seven and The Nine rings. You can find out more about them here.
PeeDee
Thanks for the link (I think!). Reading that, and the parts of the article I looked at earlier, makes me feel glad that I’ve never read, seen or listened to any of Tolkein’s output or its various interpretations.
Thanks Chalmie and PeeDee
Agree that the tough ones were tough !!! Managed all but two – couldn’t parse SNEEZE (and don’t think that I could ever have) and had an unparsed RUNTS in at 6d – no idea about the seven rings for the DWARVES.
Found it a hard one to get going on, but once started the words seemed to flow in quite constantly until reaching that north east corner. Really liked the way that both the 7 and the cathedral themes were woven into the crossword.
Finished with PUNITIVE (which I struggled to equate to ‘cruel’ to be honest), RINGS (which I got wrong) and the unheard of RECUSE.
Thanks, guys. Perversely, I got the NE first — it was SW where I trailed. It was a bit of a beast today — thanks, Chalmie, keep them coming.
Quite a challenge, but an enjoyable one at that. I, too, appreciated the interweaving of the two themes & actually spotted them quite quickly (wonders will never cease!). Favourite clue was 4d.
Thank you Chalmie and PeeDee.
Thanks pd and commenters.
The genesis of this puzzle was that it was my 343rd non-specialist puzzle. 343 is 7x7x7, so I decided to have three different sevens – once as a solution, once as a gateway, and then some uses of it as a number. I’m gratified to know that the interweaving of them gave at least some solvers some enjoyment.
Thanks to Chalmie and PeeDee.
I thought in 5 it was p=quiet, plie= (ballet) position, no?
Hi Agentzero – that works too.
Thanks Chalmie and PeeDee (and Gaufrid).
This was tough and I failed to parse both RINGS and FREEZE so your explanations are much appreciated.
I half myself up for a while at 21ac. I originally put SHIP as in S + HIP (very popular) – Ship as in “Ship Out”! Fortunately realised the error of my ways in time.
Interesting theme and particularly given the explanation from Chalmie above so thanks again.