All but one of the unclued lights can be preceded by the same word, large or small. Solvers are requested to highlight the other thematic solution which does not follow this pattern. Brewer confirms the theme.

Across

3    Acrylic pot he refashioned in copper and iron compound (12)
11    33 returning after the usual time (4)
12    Accountant’s car by hill (7)
16    Educated, but lacking a CD for reviewing piece of music (5)
18    Not much life in a cell? (6)
20    Grim heartless Ron set out first (5)
23    Sleep in it, though it’s really not fitting (5)
25    Record gold obstacle as US ornament (7)
26    Another...