Mass meeting consumes energy, in fact (6)
I believe the answer is:
really
'in fact' is the definition.
('really' can be a synonym of 'in fact')
'mass meeting consumes energy' is the wordplay.
'mass meeting' becomes 'rally' ('rally' can be a synonym of 'mass meeting').
'consumes' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'rally' enclosing 'e' is 'REALLY'.
(Other definitions for really that I've seen before include "Well I never!" , "Honestly, it's a fact" , "surely not?" , "as a matter of fact" , "In actual fact" .)