I believe the tense is incorrect in the clue for 17d.Perhaps it should read 'Happen(s) to give off water vapour' for the answer to agree.
Thanks to Lie-in-King for the explanation of 25a which I had guessed from the checking letters but was to lazy-minded to pursue. Had ARCHER for 21d until 27a in its obviousness led to changing 21d to BRAHMS.
I had a lovely time, thank you.