Standard apparently not observed? (8,4)
I believe the answer is:
anything goes
'standard' is the definition.
(jazz standard by Cole Porter)
I can't explain the rest of the clue.
(Other definitions for anything goes that I've seen before include "West End musical" , "anarchy?" , "musical - with 17a" , "Musical - statement of permissiveness" , "Cole Porter song/musical" .)