Reserving energy, switch positions with unique individual (3,3)
I believe the answer is:
one off
'unique individual' is the definition.
The definition suggests a singular noun which matches the answer.
'reserving energy switch positions' is the wordplay.
'reserving' means one lot of letters goes inside another (inserted letters get reserved or contained).
'energy' becomes 'e' (physics symbol as in E=mc2).
'switch positions' becomes 'on off' (on and off are positions on a switch).
'e' put into 'onoff' is 'ONE-OFF'.
'with' is the link.
Can you help me to learn more?
(Other definitions for one off that I've seen before include "manufactured individually" , "never to be seen again" , "Imprecise estimate?" , "in a unique way" , "leaver, might you say?" .)