One making sure he doesn't waste a thing (4)
I believe the answer is:
'a thing' is the definition.
(I know that user is a type of economic agent)
'one making sure he doesn't waste' is the wordplay.
'sure' can be anagrammed to 'USER'.
However, I am not clear how this is indicated.
Can you help me to learn more?
(Other definitions for user that I've seen before include "Person employing something" , "Person who takes drugs" , "One who exploits, to be sure" , "Person taking drugs" , "Manipulator" .)