Beats packs, full of energy (6)
I believe the answer is:
creams
'beats' is the definition.
(creaming is a kind of beating)
'packs full of energy' is the wordplay.
'packs' becomes 'crams' ('cram' can be a synonym of 'pack').
'full of' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'crams' placed around 'e' is 'CREAMS'.
(Other definitions for creams that I've seen before include "biscuits" , "Takes the best" .)