Part of The Square sustainably set up for the consumer (4)
I believe the answer is:
Here is my best explanation:
'the consumer' is the definition.
(user is a kind of consumer)
'part of the square sustainably set up' is the subsidiary indication.
'part of' indicates the answer is hidden within the clue.
'set up' says the letters should be written backwards.
'resu' can be found hidden inside 'square sustainably'.
'resu' written backwards gives 'user'.
'for' is the link.
(Other definitions for user that I've seen before include "Maybe one needing pot", "junkie", "One taking advantage", "Consumer", "One who works an appliance", "Drug-taker", "With 1a [NEW] he'll need different login", "Manipulator", "Drug addict (colloq.)" and "Employer".)