Main computer network in roof space (8)
I believe the answer is:
atlantic
'main' is the definition.
(Atlantic is an example)
'computer network in roof space' is the wordplay.
'computer network' becomes 'LAN' (abbreviation for local-area network).
'in' is an insertion indicator.
'roof space' becomes 'attic' (I've seen this before).
'lan' placed inside 'attic' is 'ATLANTIC'.
(Other definitions for atlantic that I've seen before include "UK's nearest ocean" , "Sea between Old and New Worlds" , "Azores, Falklands, in it" , "New Jersey resort" , "Tilt a can at the ocean" .)