Lament overthrow of energy network (5)
I believe the answer is:
dirge
'lament' is the definition.
(synonyms)
'overthrow of energy network' is the wordplay.
'overthrow of' shows that the letters should be reversed in order (I've seen 'overthrows' mean this).
'energy' becomes 'e' (physics symbol as in E=mc2).
'network' becomes 'grid' (grid is a kind of network).
'e'+'grid'='egrid'
'egrid' written backwards gives 'DIRGE'.
(Other definitions for dirge that I've seen before include "Funereal song" , "Melancholy song" , "Keen; lament" , "Funeral lament" , "Requiem" .)