Spy involved with Rebecca in surfing milieu (10)
I believe the answer is:
cyberspace
'surfing milieu' is the definition.
(I've seen this in another clue)
'spy involved with rebecca' is the wordplay.
'involved with' indicates an anagram (involved can mean confusing or complex).
'spy' mixed up with 'rebecca' makes 'CYBERSPACE'.
'in' acts as a link.
(Other definitions for cyberspace that I've seen before include "surfing milieu" , "Virtually real environment" , "Computerised environment" , "Worldwide computer environment" , "Notional computer communication environment" .)