BBC loses case most originally see as inopportune (8)
I believe the answer is:
untimely
'inopportune' is the definition.
('untimely' can be similar in meaning to 'inopportune')
'bbc loses case most originally see' is the wordplay.
'bbc' becomes 'auntie' (I've seen this before).
'loses case' means to look at the middle letters (the case is the outside letters).
'originally' suggests taking the first letters.
'see' becomes 'ely' (example where cathedral located in diocese).
The central letters of 'auntie' are 'unti'.
The initial letter of 'most' is 'm'.
'unti'+'m'+'ely'='UNTIMELY'
'as' acts as a link.
(Other definitions for untimely that I've seen before include "Happening too soon, perhaps like death" , "Premature, too soon" , "Of death, premature" , "Happening too soon, maybe death" , "Of death, coming too early" .)