Non-orthodox observance cannot be long-winded (7)
I believe the answer is:
verbose
'long-winded' is the definition.
(long-winded can mean overly talkative or verbose)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for verbose that I've seen before include "With an excess of words" , "Garrulous" , "Talkative, given to saying too much" , "Irritatingly long-winded" , "Loquacious" .)