Fix yours truly in money (6)
I believe the answer is:
cement
'fix' is the definition.
(cementing is a kind of fixing)
'yours truly in money' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'in' means one lot of letters goes inside another.
'money' becomes 'cent' (eg denomination of dollars).
'me' put into 'cent' is 'CEMENT'.
(Other definitions for cement that I've seen before include "Seal; bonding agent" , "It's hard" , "Building material to bind things" , "Mortar; strengthen" , "It hardens when mixed with water" .)