Argument in club banker defends (6)
I believe the answer is:
debate
'argument' is the definition.
(I know that argument can be written as debate)
'club banker defends' is the wordplay.
'club' becomes 'bat' (as in sporting equipment).
'banker' becomes 'dee' (River Dee - something that banks).
'defends' means one lot of letters goes inside another (inserted letters are defended or protected).
'bat' going into 'dee' is 'DEBATE'.
'in' is the link.
(Other definitions for debate that I've seen before include "Discuss formally" , "Argue the toss" , "Formal public discussion" , "Exchange of views" , "consideration" .)