Investigation concerning main Republican backed by church (8)
I believe the answer is:
research
'investigation' is the definition.
(research is investigating something)
'concerning main republican backed by church' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'main' becomes 'sea' (the main is an old name for the sea).
'republican' becomes 'R' (abbreviation).
'backed by' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
're'+'sea'+'r'+'ch'='RESEARCH'
(Other definitions for research that I've seen before include "Examination" , "Systematic investigation to establish facts" , "Scientific study" , "enquiry" , "Fact-finding" .)