Clara's upset by rogue (6)
I believe the answer is:
rascal
'rogue' is the definition.
('rascal' can be a synonym of 'rogue')
'clara's upset' is the wordplay.
'upset' indicates an anagram.
'claras' is an anagram of 'RASCAL'.
'by' acts as a link.
(Other definitions for rascal that I've seen before include "Mischievous or cheeky child" , "Michievous person/child" , "Impudent child" , "mischief-maker" , "Mischievous person or confused lascar" .)