I can only see 15 definition-only clues....maybe 11a ‘Wallop maker’ is one, and then 18a as suggested by you, always puzzled.
But I don’t see the answer for 18a.
Lumen, 11 A is one. 18 A is also a treated entry, though there's wordplay in it. To be properly thematic, 'French province' would have done as the clue.
Aa Lumen suggested 18a is one of the 17 but is seems to be a (helpful) hybrid. The definition needs treating thematically but the wordplay gives the modified form. That helped me,along with 20d, to find out what needed doing to the definition-only clues.I can relate to the title but do not fancy grid staring for the theme as it is now unnecessary.