Regular design of coin in Panama perhaps (9)
I believe the answer is:
canonical
'regular' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'design of coin in panama perhaps' is the wordplay.
'design of' indicates an anagram (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'panama perhaps' becomes 'canal' (I can't justify this - if you can you should give a lot more credence to this answer).
'coin' with letters rearranged gives 'onic'.
'onic' inserted into 'canal' is 'CANONICAL'.
Can you help me to learn more?
(Other definitions for canonical that I've seen before include "Accepted as being authoritative" , "Standard" .)