Sellers wanting source of valuable drug back (7)
I believe the answer is:
endorse
'drug back' is the definition.
'endorse' can be an answer for 'back' (back can mean to support or endorse). I am not sure about the 'drug' bit.
'sellers wanting source of valuable' is the wordplay.
'sellers' becomes 'vendors' ('vendor' can be a synonym of 'seller').
'wanting source' means to remove the first letter.
'of valuable' becomes 'e' (I am not sure about this - if you are sure you should believe this answer much more).
'vendors' with its first letter taken away is 'endors'.
'endors'+'e'='ENDORSE'
(Other definitions for endorse that I've seen before include "Back, support" , "Approve - support" , "Give support or approval to" , "Affirm" , "Give public approval of (something)" .)