[Pervinca Took]

1. Thank you, nurse!’ (American text speak). Here, a twisting cave leads to the beginning of the underworld.
2. Nimble Field Marshal gains direction even as all becomes confused here.
ERESTOR: See him before the decay returns; a sibilant is swallowed.
4. Ask Debussy when it sounds like he sings. It might be after noon.
ERIADOR: Thingol’s land is transformed into another, as a fricative element gives way to hesitation in the general turmoil.
6. He’s a lover of confectionery, with only one small exception.
7. Food guilt? It’s confusing when someone lays that on you.
8. His table manners leave much to be desired.
WITHYWINDLE: Shrivel without hesitation, here in France? Decline, after first loss. But on it flows.
10. Fifty pence pieces can really weigh your pockets down, I hear! That considered, on it runs.
11. Here you’ll find a stove with tooth marks in it, they say!
12. Girl traps boy here. Mother too, initially, and first.
13. Here find oriental bread, lose two articles, and meet a physician.
14. Flatulent and sibilant children’s author? (Forget his surname). It's confusing, but perhaps he’s sampled this?
15. Spanish greeting for Balin’s directionless dad; despite the confusion, it’s not that far from where his son lies.
16. Charlatan of a professor – looks peaky after much confusion!
HALFAST: He’s semi-sedentary, with very minimal disturbance.
18. An eternally tough dancer, we hear.
RAUROS: Dark Lord changes direction as he tumbles here.
20. What Drogo might have yelled here, in 2980, had he been a Cockney?
NOB: Does he really ban the use of a certain plosive?
22. To a linguist’s mind, he seems almost to specialise in golden tautology.

Perfect reasoning for ERESTOR.

Clue 17 indicates a guy who is HALF SAT, which, with minimal disturbance, does give us HALFAST.

The element you need for 15 is HOLA, not vale.