Proper bit of flagellation's upsetting (5)
I believe the answer is:
legal
'proper' is the definition.
(thesaurus)
'bit of flagellation's upsetting' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'upsetting' says the letters should be written backwards.
'lagel' is hidden within 'flagellation'.
'lagel' back-to-front is 'LEGAL'.
(Other definitions for legal that I've seen before include "Lawful" , "Relating to jurisprudence" , "Allowed by law" , "Legitimate" , "Permitted by law" .)