Essential to save energy (4)
I believe the answer is:
bare
'essential' is the definition.
(thesaurus)
'save energy' is the wordplay.
'save' becomes 'bar' (I've seen this before**).
'energy' becomes 'e' (physics symbol as in E=mca).
'bar'+'e'='BARE'
'to' is the link.
(Other definitions for bare that I've seen before include "Stark - naked" , "Unadorned - naked" , "Mere" , "Without protective covering" , "Not clothed" .)