I have completed the crossword, but am unsure of how a couple of answers are parsed:
1d) I have an answer for "unusual". But that's as far as I get with this clue.
18d) I have never heard of "S" being used as an abbreviation in this way.
mikey, 1d: defn. "unusual", parses as a 4-letter word for "Challenge" with the first letter switched, the letter switch involves abbrevs for the 2 major US political parties.
Ditto on 18d, despite being American that was my LOI by leaps and bounds.