I am pretty sure of the S The clue is art is terribly well in part of the house The answer I have is stairwell which is an anagram of artis followed by well =healthy
It will be isn't.
The clue was probably something like
" an abbreviation for 'not being' " So ISN'T seems okay.
Your paraphrased version of the clue is misleading- it read to me as if you were asking for a word that meant "not being an abbreviation"
Was there a comma in the clue then?
"Word for not being, an abbreviation". If so then ISN'T is okay.
Is it a cryptic crossword? If so then the clue is okay for ISN'T