Gravity-free network (4)
I believe the answer is:
grid
'network' is the definition.
(grid is a kind of network)
'gravity-free' is the wordplay.
'gravity' becomes 'g'.
'free' becomes 'rid' ('rid' can be a synonym of 'free'**).
'g'+'rid'='GRID'
(Other definitions for grid that I've seen before include "Map reference system" , "Electricity network" , "this 15x15 space?" , "crossword, but no clues" , "Network of crossing parallel lines" .)