Trade in treated lead (4)
I believe the answer is:
deal
'trade in' is the definition.
(thesaurus)
'treated lead' is the wordplay.
'treated' indicates an anagram.
'lead' is an anagram of 'DEAL'.
(Other definitions for deal that I've seen before include "understanding" , "Pass around (e.g. cards)" , "Large quantity (of wood?)" , "Plank of softwood timber" , "Put cards on table" .)