Religious in abundance, but dismissing heads of Catholic order (5)
I believe the answer is:
pious
'religious' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pious that I've seen before include "Godly" , "Dutiful" , "God-fearing" , "Devout" , "Self-righteous" .)