Old European prohibition restricting the players switching codes in past (6)
I believe the answer is:
theban
'old european prohibition restricting the players switching' is the definition.
I can't judge whether this definition defines the answer.
'codes in past' is the wordplay.
'codes' becomes 'ba' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'past' becomes 'then' ('then' can be similar in meaning to 'past').
'ba' inserted inside 'then' is 'THEBAN'.
Can you help me to learn more?
(Other definitions for theban that I've seen before include "Inhabitant of an ancient Egyptian capital" , "Old Egyptian" , "Old citizen" , "Relating to an ancient Greek city" , "Nab the person from the ancient Greek city" .)