Discharge key errand (8)
I believe the answer is:
emission
'discharge' is the definition.
(I know that discharge can be written as emission)
'key errand' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'errand' becomes 'mission' (both can mean a task).
'e'+'mission'='EMISSION'
(Other definitions for emission that I've seen before include "A giving out or putting forth" , "Going out" , "radiation leakage?" , "issue" , "Process of giving out" .)