Boxes with energy, being thin (6)
I believe the answer is:
sparse
'being thin' is the definition.
('sparse' can be a synonym of 'thin')
'boxes with energy' is the wordplay.
'boxes' becomes 'spars' (sparring is a kind of boxing).
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'spars'+'e'='SPARSE'
(Other definitions for sparse that I've seen before include "Thinly populated" , "Not much" , "Thinly scattered, not dense" , "In short supply" , "Thinly dotted about" .)