School circles used to be supplied with juice (7)
I believe the answer is:
powered
'supplied with juice' is the definition.
(juice can mean electrical power)
'school circles used to be' is the wordplay.
'school' becomes 'pod' (term for a group of marine mammals).
'circles' means one lot of letters goes inside another.
'used to be' becomes 'were' (eg 'you were...' means 'you used to be...').
'pod' going around 'were' is 'POWERED'.
(Other definitions for powered that I've seen before include "Supplied the force to function" , "Forcible" , "Supplied with energy" , "Like anything with an engine" , "fed juice" .)