I had no luck with trying alphabetical choices, so shelved it for the moment.Went to it this morning, looked again at down clue to get first letter, realised that answer was STUDIOUS giving D for the first letter of 21 across .. so then DOMINION
I had thought of STUDIOUS but only worked out why just now!
The M of 'dominion' gives TEAMSTER for:
Truck driver - one trying to go round America (

-
TESTER with AM in the middle?
Some connection with a film or something? Anyway, it fits!
five empty squares to fill.
12 across: TV presenter in cast for folk song (6) C - A - T -
CHANTS? I don't know, but there was something you said a while ago about a word that occurs in reference to folk songs...
25 across: One bound to work swears regularly and loudly (4) S - R - SERF? but I can't quite see why! I suppose R for regularly and F for the swearing, but sounds a bit far-fetched.