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)

...