Spectator # 2475

[Matakari]

Some answers for the following to set me on the right track, please! Thanks in advance!
ACROSS:
11 Mercury, say, in the sky (7) : ???N?NT
12 Joanna shrinks following advance of insects (5) : ???A?
15 Bug and grub infesting section of plants (7) : ???R???
23 Room behind marble corridor (8) :A??????? ( anteroom?)
38 Drop off withdrawn being outside animal park (6) : ??O??E
45 Poor Ian renovated vestibules (7) : P??N??I
DOWN:
9 Corot painted Turner perhaps twisting part of mouth (9, two words) : ??????O??
16 Fizzy drink some shake first (5): ????N
18 Lass has boot short of width (5) : E????
22 Implicitly red jam left inside steward’s room (9, two words) : ??E???A?U

[Toorak]

12 apian - a (advance) pian(o) - Joanna shrinks

[aelmpvw]

45 pronaoi

[Toorak]

18 (w)Ellie

[Toorak]

38 snooze - ens (being) round zoo all reversed

[shaneystar2]

15 Taproot
16 Kefir..... in the clue
22 Glory hole
23 Alleyway
9 Root canal

[Calibax]

11a Element

[Neveracrossword]

Hope those got you on 'the right track', Matakari - the home straight is probably in sight now!

[Matakari]

Thanks,Shaneystar2, Toorak, Aelmpvw and Calibax, for the invaluable and insightful help. This should help me to explore the poem with great excitement.