Mournful song about the first electronic network (5)
I believe the answer is:
dirge
'mournful song' is the definition.
(piece of music lamenting the dead)
'about the first electronic network' is the wordplay.
'about' shows that the letters should be reversed in order.
'the first electronic' becomes 'e' (1st letter of 'electronic').
'network' becomes 'grid' (grid is a kind of network).
'e'+'grid'='egrid'
'egrid' backwards is 'DIRGE'.
(Other definitions for dirge that I've seen before include "lamentable composition" , "Funeral lament" , "Dragging piece" , "Funeral song" , "Keen; lament" .)