Oddly crumby energy block (4)
I believe the answer is:
cube
'block' is the definition.
(cube is a kind of block)
'oddly crumby energy' is the wordplay.
'oddly' indicates alternate letters (letters in odd-numbered positions).
'energy' becomes 'e' (physics symbol as in E=mc2).
The alternate letters of 'crumby' are 'cub'.
'cub'+'e'='CUBE'
(Other definitions for cube that I've seen before include "Three-dimensional shape with six sides" , "Die" , "Six-faced solid" , "nine?" , "Geometrical solid" .)