Cryptic crossword No 3,857

Across

1. Royal Society guards are on top of that (4)
2. Layer pieces of nylon and dye (5)
3. An unopened doorway of main passageway (6)
4. Knocks down storeys of a building (6)
5. Mark free of teaching prior morning test done out loud (4,4)
6. Muscle in stew char once (8)
7. Toward the back of a grave (6)
8. Study Tesla's depression (4)
9. Concentrated hard (5)
10. Container holding no more whitewash (5-2)
11. Dance move from dance featuring ten moves at ease towards the end (3,4)
12. Drop tablet, cracks severely (4)
13. Shooter regularly squashes rolls (4)
14. Retie or at first loosen atrocious loafer (8)
15. Nut oozes liquid drops (5,3)
16. Beast in progress to destroy Puerto Rico (6)

Down

1. Fools judge briefly (5)
2. Overdose on drug to rejoice in Beethoven's song (3,2,3)
3. Transfer excavated bygone animal often in museum piece (1-3)
4. Deface country club with overwhelming welcome (8)
5. Point-blank letter framed in embellishment found on person (6)
6. What a minor assault might say about your character? (6)
7. When I travel without end, I write (6)
8. Initially eats clearly revolting unsweetened oatmeal (4)
9. Pepper dip eaten by Jack with bits of almonds laid neatly over (8)
10. Ran around atop deck occasionally (4)
11. Messily scores a goal at last in sport (8)
12. Working hard and ready (2,4)
13. Nurse masking right rage (5)
14. Scrap of Gothic spear and cannon (6)
15. White hot blade (4)
16. Kitchen counter is secure (6)