After four, Roy circulated banned substance (5)
I believe the answer is:
ivory
'banned substance' is the definition.
'ivory' can be an answer for 'substance' (ivory is a kind of substance). I'm not certain of the 'banned' bit.
'after four roy circulated' is the wordplay.
'after' means one lot of letters go next to another.
'four' becomes 'IV' (Roman numerals).
'circulated' indicates anagramming the letters.
'roy' with letters rearranged gives 'ory'.
'iv' put next to 'ory' is 'IVORY'.
Can you help me to learn more?
(Other definitions for ivory that I've seen before include "Smooth material of elephant tusks" , "Key perhaps" , "Elephant tusk material" , "Country on the Gulf of Guinea, a former French protectorate" , "Film director" .)