Languor shown as female setter ruined duet (9)
I believe the answer is:
lassitude
'languor' is the definition.
('lassitude' can be a synonym of 'languor')
'female setter ruined duet' is the wordplay.
'female' becomes 'lass' (lass is a kind of female**).
'setter' becomes 'i' (the person that set the crossword).
'ruined' is an anagram indicator.
'duet' anagrammed gives 'tude'.
'lass'+'i'+'tude'='LASSITUDE'
'shown as' is the link.
(Other definitions for lassitude that I've seen before include "slackness" , "Is saluted with tiredness" , "Weariness" , "30 [LETHARGY]" , "Lack of energy and enthusiasm" .)