Issue undertaking to support energy (8)
I believe the answer is:
emission
'issue' is the definition.
(I've seen this before)
'undertaking to support energy' is the wordplay.
'undertaking' becomes 'mission' (I've seen this in other clues).
'to support' means one lot of letters go next to another (in a down clue, some letters hold up others).
'energy' becomes 'e' (physics symbol as in E=mc2).
'mission' put after 'e' is 'EMISSION'.
(Other definitions for emission that I've seen before include "A giving out or putting forth" , "Sending forth" , "Release" , "Is in some (anag) - discharge" , "Process of giving out" .)