Network key, free (4)
I believe the answer is:
grid
'network' is the definition.
(grid is a kind of network)
'key free' is the wordplay.
'key' becomes 'g' (musical key).
'free' becomes 'rid' (to be free of something is to be rid of it).
'g'+'rid'='GRID'
(Other definitions for grid that I've seen before include "Electricity network" , "Power distribution network" , "entries in this" , "Network of crossing parallel lines" , "Network of intersecting lines" .)