The energy of six thousand? (3)
I believe the answer is:
vim
'the energy' is the definition.
('vim' can be a synonym of 'energy')
'six thousand?' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'thousand?' becomes 'm' (Roman numeral for a thousand).
'vi'+'m'='VIM'
'of' acts as a link.
(Other definitions for vim that I've seen before include "Informal energy or drive" , "Pizazz" , "Pep and energy" , "Energy and vigour" , "Liveliness" .)