Fifteensquared

Solving time - about half an hour in two sessions.

I started off quite quickly and had half of it done within 5 minutes, but then ground to a halt and had to think hard to get the last few. There were a few tricky words in the wordplay as well as in the grid. I’d never heard of 1dn or the tree in 7dn.

Across
1 DELIQUESCED = “Delhi quest”. I didn’t get this until I had all the crossing letters. I was trying to think of a synonym for emigrated.
9 BO(NAN)Z,A - Boz is Dickens’ occasional pseudonym, as in Sketches by Boz.
10 TU,TORE,D=last letter of landlord.
12 OSCAR - comes before Papa in the phonetic alphabet.
13 BLOC(k) - to scotch is to block. Last one I got, along with 7.
16 D.I.’S CONCERT - DIS is usually clued as “woman’s” or “princess’s” or Hell. I think this is the first time I’ve seen D.I. for Detective Inspector used.
19 AGED - Dega(s) reversed.
21 N.I.,OBE - a queen of Thebes in Greek mythology.
24 MANDALA(y) - a geometric pattern representing the cosmos, used in Eastern religions to aid meditation.
26 ROLLER,TOWEL(owlet*)

Down
1 DANGEROUS CORNER - a play by J. B. Priestley. A black spot road sign is positioned as a warning to drivers on dangerous corners in the U.K. I don’t know if there are similar signs elsewhere in the world.
2 LIND,A - Jenny Lind, a.k.a. the Swedish Nightingale, was a 19th-century opera singer.
3 QUA,KING - qua is Latin for as. I had ???KING straight away, but it took a run through the alphabet to find the right answer.
4 ESTONIA - A.I.,NOT,S(pac)E all reversed.
7 A,B,SORB - a sorb is a type of tree, which I looked up afterwards.
8 ADO,RN,S(wab)
15 D(OVET)AIL - vote* in DAIL, which is the Irish parliament.
17 C(A,SCAD)E - I think I’ve heard of a scad before as a type of fish…
20 DUDLEY - Dudley Moore and a town in the midlands.