thanks to those who answered - esp pictorially - my q about 9d in 2521 - BUT how does it work? I had 'blah' for 28a - B lah - which means nonsense, blab doesn't work for the notes, but would be fine for an unclued light! 47a must be 'sean' - a fishing net, and sea + n for new.
No four-letter solutions mentioned, and what is the -u- at the start? 'Tau' is surely right from Minotaur ...
Blah does work - B (musical note) + lah - note from tonic sol-fa
Sean is an alternative spelling of sein
Tau is right but I no longer have the puzzle so can't comment further.
Thank you, Professor Maisie - I felt it must be harpoon, but missed the entry in Chambers - then I was told it was barn owl! Hence my bafflement - Revox