Discharge from energy undertaking (8)
I believe the answer is:
emission
'discharge' is the definition.
(I know that discharge can be written as emission)
'energy undertaking' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'undertaking' becomes 'mission' (I've seen this in other clues).
'e'+'mission'='EMISSION'
'from' is the link.
(Other definitions for emission that I've seen before include "it gets thrown out" , "Is in some (anag) - discharge" , "Something given out" , "A giving out or putting forth" , "Process of giving out" .)