A fiend smashed church in opposition (8)
I believe the answer is:
defiance
'opposition' is the definition.
(I've seen this before)
'a fiend smashed church' is the wordplay.
'smashed' indicates anagramming the letters.
'church' becomes 'CE' (abbreviation for Church of England).
'a'+'fiend'='afiend'
'afiend' with letters rearranged gives 'defian'.
'defian'+'ce'='DEFIANCE'
'in' is the link.
(Other definitions for defiance that I've seen before include "Open resistance" , "Resistance to authority" , "to make a challenge" , "Bold disobedience" , "Refusal to obey" .)