Official cast regularly fails (6)
I believe the answer is:
formal
'official' is the definition.
(similar in meaning)
'cast regularly fails' is the wordplay.
'cast' becomes 'form' (synonyms).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'fails' are 'al'.
'form'+'al'='FORMAL'
(Other definitions for formal that I've seen before include "It's required by rule" , "Conforming to standards of correctness" , "conventionally correct" , "stuffy" , "Stately or official" .)