Polish Catholic tucked into tart (5)
I believe the answer is:
scour
'polish' is the definition.
(thesaurus)
'catholic tucked into tart' is the wordplay.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'tucked into' is an insertion indicator.
'tart' becomes 'sour ' (similar in meaning).
'c' put into 'sour' is 'SCOUR'.
(Other definitions for scour that I've seen before include "Clean by scrubbing" , "Rub with a gritty substance" , "Look high and low" , "Thoroughly clean" , "scrub and scrape" .)