Peculiar code, peculiar etiquette (7)
I believe the answer is:
decorum
'etiquette' is the definition.
(I've seen this before)
'peculiar code peculiar' is the wordplay.
'peculiar' is an anagram indicator (a peculiar or strange ordering of the letters).
'peculiar' becomes 'rum' (synonyms).
'code' with letters rearranged gives 'deco'.
'deco'+'rum'='DECORUM'
(Other definitions for decorum that I've seen before include "This is becoming" , "Proper behaviour; etiquette" , "good behaviour" , "Propriety of behaviour" , "Good manners" .)