Demeanour of experts circulating mails to support admission of guilt for covering up deception (15)
I believe the answer is:
professionalism
'demeanour of experts' is the definition.
(how professionals behave)
'circulating mails to support admission of guilt for covering up deception' is the wordplay.
I cannot quite see how this works, but
an anagram of 'mails' is 'alism' which is located in the answer.
an anagram of 'for' is 'rof' which is located in the answer.
'covering' could be 'on' and 'on' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for professionalism that I've seen before include "Doctor needs to show this skill" , "Competence of experts" , "Skill not displayed by amateur" .)