Efforts of heartless foes seizing empty Reichstag (8)

'efforts' is the definition.
(both can mean a person's work or actions)

'heartless foes seizing empty reichstag' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'foes' becomes 'enemies' (enemy is a kind of foe).
'seizing' means one lot of letters goes inside another.
'empty' says to hollow out the word (remove centre letters).
'enemies' with its middle taken out is 'eneies'.
'reichstag' with its centre taken out is 'rg'.
'eneies' placed around 'rg' is 'ENERGIES'.

'of' acts as a link.