ten up!

[rodwels]

The answer has to have the letters T E N in consecutive letters in that order. Clue is:
"This is something which just will not last" (9)

[pbeach]

Untenable?

[rodwels]

Thanks, I had thought of that myself but wasn't entirely convinced, but I think I'll go with it now.

I thought both transient and momentary were perhaps better answers to the clue, but they didn't have TEN in the right order.