Terms of Detroit mafia deal with Colombian -- principally to supply powder (4)
I believe the answer is:
'powder' is the definition.
(I know that talc is a more specific form of the action powder)
'terms of detroit mafia deal with colombian principally' is the wordplay.
'terms of' says to take the final letters (term can mean the end of something).
'with' is a charade indicator (letters next to each other).
'principally' suggests taking the first letters.
The last letters of 'detroit mafia deal' are 'tal'.
The first letter of 'colombian' is 'c'.
'to supply' is the link.
(Other definitions for talc that I've seen before include "Mineral used as a toiletry" , "Baby powder" , "Powdered substance" , "Drier" , "Fragrant powder" .)