Proper behaviour (7)
(Other definitions for decorum that I've seen before include "Dignity of behaviour or proceedings" , "Seemliness" , "Protocol" , "This is becoming" , "Good manners" .)
'proper behaviour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for decency that I've seen before include "Decorum" , "Goodness - sense of what is fitting" , "Respectable behaviour" , "Propriety" , "Seemliness" .)