Six-footers in religious groups? (7)
I believe the answer is:
insects
'six-footers' is the definition.
(I know this)
'in religious groups?' is the wordplay.
'religious groups?' becomes 'sects' (sect is a type of religious group).
'in'+'sects'='INSECTS'
(Other definitions for insects that I've seen before include "Small arthropods" , "Six-leggged creatures" , "Small animals" , "Six-legged creatures" , "Ants, say" .)