Join me in the money (6)
I believe the answer is:
cement
'join' is the definition.
(I've seen this before)
'me in the money' is the wordplay.
'in the' is an insertion indicator.
'money' becomes 'cent' (eg denomination of dollars).
'me' inserted within 'cent' is 'CEMENT'.
(Other definitions for cement that I've seen before include "Bind (friendship)" , "Seal; bonding agent" , "Make fast - adhesive" , "Ingredient of concrete" , "Brick fixer" .)