Don't heighten problem of acceptable behaviour (3,4,5)
I believe the answer is:
the done thing
'acceptable behaviour' is the definition.
(I've seen this before)
'don't heighten problem' is the wordplay.
'problem' indicates anagramming the letters.
'dont'+'heighten'='dontheighten'
'dontheighten' with letters rearranged gives 'THE DONE THING'.
'of' is the link.
(Other definitions for the done thing that I've seen before include "Etiquette" , "That which is conventional, acceptable and desirable" , "normal behaviour" , "Socially approved behaviour" , "Acceptable behaviour" .)