Deduce computer network turned evil (8)
I believe the answer is:
infernal
'evil' is the definition.
(similar in meaning)
'deduce computer network turned' is the wordplay.
'deduce' becomes 'infer' (synonyms).
'computer network' becomes 'LAN' (abbreviation for local-area network).
'turned' shows that the letters should be reversed in order.
'lan' backwards is 'nal'.
'infer'+'nal'='INFERNAL'
(Other definitions for infernal that I've seen before include "Hell-like" , "Devilish" , "Of hell, or tiresome like a nuisance" , "To do with hell and its fires" , "To do with hell or informally tiresome" .)