Rt19

[redglenys]

6d smell carbon over cove [5] ?r?e?
14d learning to concoct rude thing before first turning[9] ?r?d?t?o?
17d pm outside gym with European chemist's tube [7] ?I?????
21d eats onions, eh? regularly[4]
20a brief scene from Egyptian capital after careful vetting [8] ?I???t??
23a lower layers of a cake returned following payments [9[ ?u???r???

[scorpiojo]

6 C reek
14 erudition

21 n o s h

[Captain2]

6d C-reek
21d Nosh - odd letters of ;onions eh'

[Captain2]

...sorry even letters of

[Captain2]

23a Stratums
20 just answered by danny as vignette

[dannyk13]

6d.Creek
14d.Erudition
17d.Pipette
20a.Vignette

[scorpiojo]

17 Pi PE tt E

[Captain2]

Ignore 23a - I'm a letter short!

[Captain2]

23 Subtratas?

[Jackdaw33]

23 Substrata

[Captain2]

Hells teeth, I get there yet!

Substrata

[Jackdaw33]

Reverse of a tart + subs (payment).

[puzzlemaniac]

would 23a be better as substrate, rather than stratums? The plural of stratum is strata, and substrate fits the letters provided

[redglenys]

23a letters are now ?u?s?r?t? substrata?

[scorpiojo]

Strata - plural

[Jackdaw33]

tart a after subs.

[redglenys]

thank you all

[elizabethsarah]

14d - erudition? (anagram of rude and then other letters I can't fit to the clue!)
21d - nosh (alternate letters in onions Eh) eats
23a - substrata = lower soil layers - (subs =payments, followed by a tart (cake) written in reverse