Variable computer network returned, without modifications externally, in investigation (8)
I believe the answer is:
analysis
'investigation' is the definition.
(analysis is a kind of investigation)
'variable computer network returned without modifications externally' is the wordplay.
'variable' becomes 'y' (I've seen this in other clues).
'computer network' becomes 'lan' (abbreviation for local-area network).
'returned' shows that the letters should be reversed in order.
'without modifications' becomes 'asis' ('as is' can mean as things currently are).
'externally' means one lot of letters goes inside another (some letters go on the outside of others).
'y'+'lan'='ylan'
'ylan' written backwards gives 'naly'.
'naly' inserted within 'asis' is 'ANALYSIS'.
'in' acts as a link.
(Other definitions for analysis that I've seen before include "Investigation of the component parts which make up the whole" , "Dissection and examination" , "A taking apart for examination" , "See 15" , "Detailed breakdown" .)