Overturned laws barring electronic libel (4)
I believe the answer is:
slur
'libel' is the definition.
(I've seen this before)
'overturned laws barring electronic' is the wordplay.
'overturned' says the letters should be written in reverse.
'laws' becomes 'rules' (rule is a kind of law).
'barring' indicates named letters should be taken away (bar can mean keep out or exclude).
'electronic' becomes 'e' (as in e-mail or e-books).
'rules' with 'e' taken away is 'ruls'.
'ruls' in reverse letter order is 'SLUR'.
(Other definitions for slur that I've seen before include "Insinuation, allegation" , "Pronounce indistinctly or smear" , "Slight, insult" , "Defamation" , "become inarticulate" .)