Traditional congregation not beginning with fish (3-6)
I believe the answer is:
old school
'traditional' is the definition.
(I've seen this in another clue)
'congregation not beginning with fish' is the wordplay.
'congregation' becomes 'fold' (as in welcoming someone into 'the fold').
'not beginning' means to remove the first letter.
'with' says to put letters next to each other.
'fish' becomes 'school' (I can't justify this - if you can you should believe this answer much more).
'fold' with its initial letter taken away is 'old'.
'old'+'school'='OLD-SCHOOL'
Can you help me to learn more?
(Other definitions for old school that I've seen before include "Not influenced by newer styles" , "In Westminster" , "sort of tie?" , "Classic" , "Doing things the same way as before" .)