The unclued lights, when paired, are of a kind, the first word in each pair being thematic. Two of the unclued lights, one of which is plural, do double duty. One pair has a literary reference, too.
 
Across
 
5    Cross as a sign (6)
10    I left millionaire mixing petroleum distillate (10, two words)
16 What may be caught with a rod — a rod (5)
17 Strips, rubs gently and beats (7)
18 Pipe second carol outside Kings (7)
20 Those assailing verbally about to take on loathsome heartless rebels (8)
25    Just a bit of cement added (3)
26    Run faster than dismissal speed? (7)
28    Works...