Unique type of switch saving energy (3-3)
I believe the answer is:
one-off
'unique' is the definition.
(I've seen this before)
'type of switch saving energy' is the wordplay.
'type of switch' becomes 'onoff' (I can't explain this - if you can you should give a lot more credence to this answer).
'saving' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mcn).
'onoff' enclosing 'e' is 'ONE-OFF'.
Can you help me to learn more?
(Other definitions for one-off that I've seen before include "Happening only once" , "Unrepeatable experience" , "Made or done as a single thing, not repeated" , "Not repeated, being a single event or object" , "Unique thing" .)