20 across
Failure
to
notice
sensitive
tenant
is
upset
(15)
Failure to notice sensitive tenant is upset (15)
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The definition is at the beginning of the clue.
How the clue works
Look at "is upset" at the end, which is your anagram indicator, telling you to shuffle the letters of sensitive tenant until they spell out the answer. The definition is Failure to notice, which is just what inattentiveness means.