Unique energy incorporated in switch settings? (3-3)
I believe the answer is:
one-off
'unique' is the definition.
(I've seen this before)
'energy incorporated in switch settings?' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'incorporated in' indicates putting letters inside.
'switch settings?' becomes 'onoff' (the two settings of a switch).
'e' placed inside 'onoff' is 'ONE-OFF'.
(Other definitions for one-off that I've seen before include "Item made once only" , "Something unique" , "Unique person or occurrence" , "Unique thing" , "Unrepeatable experience" .)