Main network in roof space (8)
I believe the answer is:
atlantic
'main' is the definition.
(Atlantic is an example)
'network in roof space' is the wordplay.
'network' becomes 'lan' (LAN is an example).
'in' is an insertion indicator.
'roof space' becomes 'attic' (I've seen this before).
'lan' inserted within 'attic' is 'ATLANTIC'.
(Other definitions for atlantic that I've seen before include "Azores, Falklands, in it" , "UK's nearest ocean" , "'The Pond'" , "What lies to the east of 10" , "Latin cat found, strangely, in the ocean" .)