Doubter infected Catholic inside (7)
I believe the answer is:
sceptic
'doubter' is the definition.
(synonyms)
'infected catholic inside' is the wordplay.
'infected' becomes 'septic' (similar in meaning).
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'inside' is an insertion indicator.
'septic' going around 'c' is 'SCEPTIC'.
(Other definitions for sceptic that I've seen before include "He's hard to convince" , "Disbeliever" , "One habitually doubts accepted beliefs" , "One questioning accepted opinions" , "Thomas for one" .)