Discharges vacuous engineers covering important assignment (9)
I believe the answer is:
emissions
'discharges' is the definition.
('emission' can be a synonym of 'discharge')
'vacuous engineers covering important assignment' is the wordplay.
'vacuous' suggests removing the centre.
'covering' is an insertion indicator.
'important assignment' becomes 'mission' (I've seen this before).
'engineers' with its middle taken out is 'es'.
'es' placed around 'mission' is 'EMISSIONS'.
(Other definitions for emissions that I've seen before include "Gases released" , "Things given off" , "Discharges" .)