Conventional occurs regularly to Shelley, perhaps (9)
I believe the answer is:
customary
'conventional' is the definition.
(similar in meaning)
'occurs regularly to shelley perhaps' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'shelley perhaps' becomes 'mary' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The alternate letters of 'occurs' are 'cus'.
'cus'+'to'+'mary'='CUSTOMARY'
Can you help me to learn more?
(Other definitions for customary that I've seen before include "According to usual practice" , "Commonly used or practised" , "Habitual, usual" , "Courts may find this usual" , "Conventional" .)