Church office opposed to rest cure? Yes! (6)
I believe the answer is:
vestry
'church office' is the definition.
(I've seen this before)
'opposed to rest cure? yes' is the wordplay.
'opposed to' becomes 'v' (abbreviation for versus).
'cure?' is an anagram indicator (I've seen 'cured' mean this).
'yes' becomes 'y'.
'rest' anagrammed gives 'estr'.
'v'+'estr'+'y'='VESTRY'
(Other definitions for vestry that I've seen before include "Room in a church where sacred vessels and vestments are kept" , "Church robing-room" , "vicar's office" , "Small room attached to church" , "Church changing room" .)