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I found this mostly straightforward.
The exceptions were blind spots for 10A, where the answer was obvious, but it took me a while to work out the wordplay; and more particularly 16A, where the answer was not clear to me for far too long.
| Across | |||
| 1. | This grammatical unit’s length is within reason (6) | ||
| CLAUSE | An envelope (‘within’) of L (‘length’) in CAUSE (‘reason’). | ||
| 4. | Glass boat (8) | ||
| SCHOONER | Double definition. We had this word in yesterday’s Rufus. | ||
| 9. | Reach a river bend (6) | ||
| ARRIVE | An anagram (‘bend’) of ‘a river’. | ||
| 10. | Up to the time of cutting support in prison (8) | ||
| BASTILLE | An envelope (‘cutting’) of TILL (‘up to the time’) in BASE (‘support’). | ||
| 12. | Conceited artist retracted tirade over work sent back (8) | ||
| ARROGANT | An envelope (‘over’) of OG, a reversal (‘sent back’) of GO (‘work’) in AR, a reversal (‘retracted’) of RA (‘artist’) plus RANT (‘tirade’). | ||
| 13. | In favour of putting on spelling contest at an earlier time (6) | ||
| BEFORE | An envelope (‘putting on’) of FOR (‘in favour of’) in BEE (‘spelling contest’). | ||
| 15. | Leisure facility (4) | ||
| EASE | Double definition. | ||
| 16. | It’s extremely beneficial to cover everything with glass (7) | ||
| BALLOON | An envelope (‘to cover’) of ALL (‘everything’) in BOON (‘it’s extremely beneficial’), for a large wineglass or brandy snifter. | ||
| 20. | Grazing land beyond a river (7) | ||
| PASTURE | A charade of PAST (‘beyond’) plus URE (‘a river’). | ||
| 21. | Blue feathers (4) | ||
| DOWN | Double definition. | ||
| 25. | Right back crosses one to get a goal (6) | ||
| TARGET | An envelope (‘crosses’) of A (‘one’) in TR, a reversal (back’) of RT (‘right’) plus ‘get’. | ||
| 26. | Set off for an appointment involving school (8) | ||
| DETONATE | An envelope (‘involving’) of ETON (‘school’) in DATE (‘appointment’). | ||
| 28. | Herb is 12 amazingly (8) | ||
| TARRAGON | An anagram (‘amazingly’) of ARROGANT, the answer to 12A. | ||
| 29. | A warning for Mark (6) | ||
| NOTICE | Double definition. | ||
| 30. | Talk with the Head of Charterhouse about poetry (8) | ||
| CONVERSE | A charade of C (‘head of Charterhouse’) plus ON (‘about’) plus VERSE (‘poetry’). | ||
| 31. | Leave dead trees all over the place (6) | ||
| DESERT | A charade of D (‘dead’) plus ESERT, an anagram (‘allover the place’) of ‘trees’. | ||
| Down | |||
| 1. | Absurd behaviour before Spurs’ opening game (8) | ||
| CHARADES | A charade (what else?) of CHARADE (‘absurd behaviour’) plus S (‘Spurs opening’). | ||
| 2. | Improve the image of Belgium after broadcast on race (8) | ||
| AIRBRUSH | A charade of AIR (‘broadcast’) plus B (‘Belgium’, IVR) plus RUSH (‘race’). | ||
| 3. | Wild goat oddly raised in keep (6) | ||
| SAVAGE | An envelope (‘in’) of AG, a reversal (‘raised’ in a down clue) in SAVE (‘keep’). | ||
| 5. | Talk about those who volunteer after church (4) | ||
| CHAT | A charade of CH (‘church’) plus AT, a reversal (‘about’) of TA (‘those who volunteer’). | ||
| 6. | Not surprisingly he gets hit at exactly the arranged time (2,3,3) | ||
| ON THE DOT | A charade of ONT, an anagram (‘surprisingly’) of ‘not’ plus ‘he’ plus DOT (‘hit’). | ||
| 7. | Sailor discovered in a hold (6) | ||
| NELSON | Double definition. | ||
| 8. | Staggered by revolutionary capturing general in retreat (6) | ||
| REELED | An envelope (‘capturing’) of EEL, a reversal (‘in retreat’) of LEE (Robert E, ‘general’) in RED (‘revolutionary’). | ||
| 11. | Dig up body part during organised hunt (7) | ||
| UNEARTH | An envelope (‘during’) of EAR (‘body part’) in UNTH, an anagram (‘organised’) of ‘hunt’. | ||
| 14. | In a panic when Capone approaches carrying a gun (7) | ||
| ALARMED | A charade of AL (‘Capone’) plus ARMED (‘carrying a gun’). | ||
| 17. | Cut material at speed (8) | ||
| LACERATE | A charade of LACE (‘material’) plus RATE (‘speed’). | ||
| 18. | It’s used in calling for the return of Peron caught earlier on the outskirts of Valparaiso (8) | ||
| VOCATIVE | A charade of VO (‘the outskirts of ValparisO‘) plus C (‘caught’) plus ATIVE, a reversal (‘the return of’) of EVITA (‘Peron’). | ||
| 19. | Benefit from a hobby (8) | ||
| INTEREST | Double definition. | ||
| 22. | Still with French film star in South Carolina (6) | ||
| STATIC | An envelope (‘in’) of TATI (Jacques, ‘French film star’) in SC (‘South Carolina’). | ||
| 23. | Finished speaking about lines by king in rehearsal (3,3) | ||
| DRY RUN | An envelope (‘about’) of RY (railway ‘lines’) plus R (‘king’) in DUN, a homophone (‘speaking’) of DONE (‘finished’). | ||
| 24. | Sullen fictional detective embracing love (6) | ||
| MOROSE | An envelope (’embracing’) of O (‘love’) in MORSE (‘fictional detective’). | ||
| 27. | Abraham’s nephew initially struck a great deal (4) | ||
| LOTS | A charade of LOT (‘Abraham’s nephew’) plus S (‘initially Struck’). | ||
Although it did tend towards the chestnutty side of clue/solution, I did enjoy myself so thank you Peto and PeterO.