Cuts across half of such faith groups (5)
I believe the answer is:
sects
'cuts' is the definition.
Although both the answer and definition are plural nouns, I cannot see how one could define the other.
'across half of such faith groups' is the wordplay.
'across' means one lot of letters goes inside another.
'half of such faith' becomes 'c' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'groups' becomes 'sets' (a set is a group of things).
'c' going within 'sets' is 'SECTS'.
Can you help me to learn more?
(Other definitions for sects that I've seen before include "Orders" , "Religious factions" , "Groups of people with unconventional religious beliefs" , "Schools of thought" , "Dissenting cliques" .)