Occurring at intervals (8)
'occurring at intervals' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for periodic that I've seen before include "Happening at intervals" , "At fixed intervals" , "Occasional" , "every now and then" , "Occurring or appearing at intervals" .)
(Other definitions for episodic that I've seen before include "taken one piece at a time" , "Is copied in irregular occurrence" , "in serial form" , "piecemeal" , "Divided into tenuously related parts" .)