Terry
I agree all possible info should be given, when posting a request for help here, and perhaps I should have waited until Pezbuin came back with the answer to 5dn. However I have seen this particular clue before in one form or another where the name of the union is related to some form of being at one.
So. trade union = unison. People playing music as one are in unison. fair enough.
Please forgive my little dig. Nothing personal meant. I tend to have a little rant now and again.
No probs. Didn't take it as a dig, rather more a reminder, that if I want help with something, then I should supply the correct info first. It was my genuine mistake.