I've taken over sanctum on church grounds (8)
I believe the answer is:
evidence
'grounds' is the definition.
(synonyms)
'i've taken over sanctum on church' is the wordplay.
'taken over' says the letters should be written backwards.
'sanctum' becomes 'den'.
'on' means one lot of letters go next to another.
'church' becomes 'ce' (abbreviation for Church of England).
'ive' in reverse letter order is 'evi'.
'evi'+'den'+'ce'='EVIDENCE'
(Other definitions for evidence that I've seen before include "Confirmation" , "Information to help form a conclusion" , "Knowledge on which to base belief" , "Information in a law case" , "authentication" .)