Order key poem. It regulates what may be worn (5,4)
I believe the answer is:
dress code
'it regulates what may be worn' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'order key poem' is the wordplay.
'order' becomes 'dress' (I can't justify this - if you can you should give a lot more credence to this answer).
'key' becomes 'c' (musical key).
'poem' becomes 'ode' (ode is a type of poem).
'dress'+'c'+'ode'='DRESS CODE'
Can you help me to learn more?
(Other definitions for dress code that I've seen before include "Convention governing clothes to be warn on a particular occasion" , "Guidelines" , "Stipulated attire" , "Expected standard of attire" .)