Is to take legal action against children (5)
I believe the answer is:
issue
'children' is the definition.
(thesaurus)
'is to take legal action against' is the wordplay.
'to take legal action against' becomes 'sue' (I've seen this before).
'is'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "Publish; children" , "Topic; give out" , "Emanate" , "Give out; children" , "Edition of a publication" .)