Secure network beginning to decay (4)
I believe the answer is:
land
'secure' is the definition.
(I've seen this in other clues)
'network beginning to decay' is the wordplay.
'network' becomes 'lan' (LAN is an example).
'beginning to' suggests taking the first letters.
The first letter of 'decay' is 'd'.
'lan'+'d'='LAND'
(Other definitions for land that I've seen before include "Terrain" , "Women farm workers in World War II" , "Area of ground" , "win" , "part of surface" .)