One making sure he doesn't waste a thing (4)

I believe the answer is:

user

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.

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