Clara's upset by a rogue (6)
I believe the answer is:
rascal
'a rogue' is the definition.
(synonyms)
'clara's upset' is the wordplay.
'upset' indicates anagramming the letters.
'claras' is an anagram of 'RASCAL'.
'by' acts as a link.
(Other definitions for rascal that I've seen before include "Brat" , "A mischievous and maybe dishonest person" , "Affectionate term for imp" , "naughty child" , "wretch" .)