Confused us with details in tiredness (9)
I believe the answer is:
lassitude
'tiredness' is the definition.
(thesaurus)
'confused us with details' is the wordplay.
'confused' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'us' put after 'details' is 'detailsus'.
'detailsus' anagrammed gives 'LASSITUDE'.
'in' is the link.
(Other definitions for lassitude that I've seen before include "Listlessness, state of apathy" , "Listlessness, lethargy" , "Weariness" , "slackness" , "Weakness" .)