To publish is to take legal action (5)
I believe the answer is:
issue
'to publish' is the definition.
('issue' can be a synonym of 'publish')
'is to take legal action' is the wordplay.
'to take legal action' becomes 'sue' (I've seen this before).
'is'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "Outcome - discharge" , "Subject for discussion; dish out" , "Emerge" , "Edition of a publication" , "Point in dispute" .)