The unclued lights, in one case paired, are all suggested by a thematic phrase (two words), which is set out in an appropriate form in the grid and must be highlighted.

Across

1 Brass and slate vessel (8)
6 Really humid round townships (6)
10 Ham cast chose plays — they cause discomfort (12)
12 Society cuts old displays (5)
13 Ape-like evangelical with outstanding pledges (7)
15 Beer and staple food one leaves in wood (6)
17 No gin in a sozzled artist (8)
20 Picture takers like river in Iceland (8)
22 Draw close, at first, in nocturnal habit (7)
24 See cuckoos and dogs (5)
26 Overcook, taking week...