1960s rock band's stuff filled with energy (5)
I believe the answer is:
cream
'1960s rock band's' is the definition.
I can't tell whether this defines the answer.
'stuff filled with energy' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'filled with' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mcr).
'cram' going around 'e' is 'CREAM'.
Can you help me to learn more?
(Other definitions for cream that I've seen before include "Butterfat part of milk" , "Top or best part" , "Thick milk" , "An off-white shade" , "It rises to the top, the best" .)