Strange code, strange etiquette (7)
I believe the answer is:
decorum
'etiquette' is the definition.
(I've seen this before)
'strange code strange' is the wordplay.
'strange' is an anagram indicator.
'strange' becomes 'rum' (synonyms).
'code' anagrammed gives 'deco'.
'deco'+'rum'='DECORUM'
(Other definitions for decorum that I've seen before include "modesty" , "Correct behaviour" , "Good manners" , "good grace" , "Decency" .)