The unclued entries, all across, are unconnected words from Chambers. Each requires a letter added to render them thematic, and these letters, in order, give a hypothetical venue where they could all then meet.

Across

8 Give a hand to a grand opera (4)
11 Where Raleigh studied El Greco, and Ollie messed about (12, two words)
16 Measure things expired audibly (4)
17 Journey eternally? (5)
18 Stupor in European Parliament – it shows social distancing? (6)
22 Bird hiding head needs rear visor (8)
24 Fools following rejected fish fingers? (6)
25 Editor fixed electrical component (6)
29 In a row, sent back nothing before breakfast?...