Unique kind of switch conserving energy (3-3)
I believe the answer is:
one-off
'unique' is the definition.
(I've seen this before)
'kind of switch conserving energy' is the wordplay.
'kind of switch' becomes 'on off' (I've seen this in another clue).
'conserving' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'onoff' going around 'e' is 'ONE-OFF'.
(Other definitions for one-off that I've seen before include "Made as single article, not repeated" , "Item made once only" , "Something unique" , "Unique specimen" , "Unique person or occurrence" .)