Vaguely pious restraining of French religious group (4,3)
I believe the answer is:
opus dei
'religious group' is the definition.
(I've seen this before)
'vaguely pious restraining of french' is the wordplay.
'vaguely' indicates an anagram.
'restraining' is an insertion indicator.
'of french' becomes 'de'.
'pious' anagrammed gives 'opusi'.
'opusi' enclosing 'de' is 'OPUS DEI'.
(Other definitions for opus dei that I've seen before include "ritual worship" , "RC organization which maintains, among others, the University of Pamplona" , "RC organisation which maintains various universities" , "Group of believers" , "prayers" .)