Six unclued lights are 18 Down from the same source (five are entitled to appear; the sixth appears regularly). Six cells initially contain between two and six clashing letters; these must be resolved to display thematic 18. Elsewhere, ignore three accents and two apostrophes.
 

Across

 
10    A Republican politician maintains British dealt badly with defence material (12, hyphened)
12    Marsh tree (5)
13    In speech, lifts poet’s handles (7)
15    Awkwardly got up on camel bred to travel (9)
16    Crack plus heroin – a drug (6)
17    Protecting money, American wealthy mostly lacking heart? (8)
21    With musical backing, orator’s clipped style...