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Given the shape of the octagon and the spelling out of SVENGALI, I assumed it was Trilby too.

No. It's something by an Italian.

Thanks barry. will have to look again.
So the clashing letters spell out another name?
Is it an opera?

Yes it's an opera based on a play of same name

any chance of one last hint before I give up and get ready for tomorrow?

It's another type of hat.

Princess is the unaltered clue,

Many thanks

many thanks barryjbutler and djawhufc, very clever I wonder how many others latched onto 'svengali' like me and didnt let go,
thanks again (although I had already posted wrong solution)

http://www.oxforddictionaries.com/us/definition/american_english/taenia

See also 30a in The Listener Crossword on the same day:

Moreover in Perth, Scots own reflecting headband (6)