Old judge — part of nine to uphold and issue a ban (6)
'a ban' is the definition.
'old judge part of nine to uphold' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'judge' becomes 'j'.
'part of' means one lot of letters goes inside another (part of the inside).
'to uphold' shows that the letters should be reversed in order (I've seen 'upheld' mean this).
'oj' put into 'nine' is 'niojne'.
'niojne' in reverse letter order is 'ENJOIN'.
'and issue' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for enjoin that I've seen before include "Instruct or warn", "Command or forbid", "Impose an order upon", "Instruct (someone to do something)", "Direct with authority".)