Unclued lights (including two of two words) suggest nine words (all in Chambers) starting with the same four-letter word that will appear diagonally in the completed grid and must be shaded.
Across
1 A grass posing as gulfweed (8)
5 Dip in swirling river (3)
13 Oxford college with pool and some land (7)
15 Refuse in Cornish river (5)
16 Goats from Croatia within Aussie island (5)
17 Lustred cloth pressed by a painter? (6)
18 Silver and bird get on together (5)
20 Most rum in hot toddies I spurned (6)
21 Almost like Hamlet’s hairs (5)
22 I blend fine rum mixing one for madam (7)
27 Female convict meets hard woman (7)
29 See red and shiny black (5)
30 Safe Marley loaded (6)
32 Lively tune keeps one merry (5)
34 Slag crude Corsican stripped (6)
36 Curses from mother not specified (5)
37 The Gallic gentleman returned Georgia’s sugar (5)
38 Repellence of strong drink is my dogma (7)
39 Country garden east of empty fish pond (6)
40 Composer is retiring, we hear (3)
42 No agency managed compound (8)
Down
2 Clique stalk hawk (8)
4 Camel masters cooked shellfish (12, two words)
5 Poop? Standard fertilizer (7)
6 New larder contains iron shelving (8)
8 What could be straw roof covering (4)
9 Cop destroyed Nazi poem (6)
10 Two erks climbing ordinary tree (5)
12 I weaken first getting ringworm (6)
21 Rector in Spain breaking rib (9)
23 Odd tailor perhaps clothing bizarre rake (8)
24 Smashing prince with spirit cycling (8)
25 Horse dad found in lake (6)
26 Deeds initially in a prescribed file (7, two words)
28 Lasses promoting recipe start to eat fish (6)
31 Distance no jay flies (5)
33 Nigerian crosses river by Scottish suburb (5)
A first prize of £30 for the first correct solution opened on 2 May.

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