14 across
Are
fired
(sacked),
being
thin
(8)
Are fired (sacked), being thin (8)
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The definition is at the end of the clue.
How the clue works
Right, so "being thin" is your definition there, pointing to rarefied, as in low density. Now look at "Are fired" — "sacked" is telling you to shake those letters up, an anagram that rearranges to give RAREFIED.