Lethargy caused by us? Details confused (9)
I believe the answer is:
lassitude
'lethargy' is the definition.
('lassitude' can be a synonym of 'lethargy')
'us? details confused' is the wordplay.
'confused' indicates anagramming the letters.
'us'+'details'='usdetails'
'usdetails' is an anagram of 'LASSITUDE'.
'caused by' acts as a link.
(Other definitions for lassitude that I've seen before include "Lack of energy and enthusiasm" , "Is saluted with tiredness" , "Listlessness, state of apathy" , "slackness" , "Weakness" .)