A way with money at last, but a lack of interest (6)
I believe the answer is:
apathy
'a lack of interest' is the definition.
(thesaurus)
'a way with money at last' is the wordplay.
'way' becomes 'path' (path is a kind of way**).
'with' says to put letters next to each other.
'at last' suggests the final letters.
The final letter of 'money' is 'y'.
'a'+'path'+'y'='APATHY'
'but' is the link.
(Other definitions for apathy that I've seen before include "Absence of emotion or enthusiasm" , "Torpor" , "Flatness" , "Detachment" , "lack of involvement" .)