Clara's upset by the scoundrel (6)
I believe the answer is:
rascal
'the scoundrel' is the definition.
(rascal is a kind of scoundrel)
'clara's upset' is the wordplay.
'upset' is an anagram indicator.
'claras' with letters rearranged gives 'RASCAL'.
'by' acts as a link.
(Other definitions for rascal that I've seen before include "A mischievous and maybe dishonest person" , "Michievous person/child" , "Scallywag" , "A naughty chap or bold boy" , "Brat" .)