Setter and mistress finally take legal action against the children (5)
I believe the answer is:
issue
'the children' is the definition.
(thesaurus)
'setter and mistress finally take legal action against' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'and' says to put letters next to each other.
'finally' suggests the final letters.
'take legal action against' becomes 'sue' (I've seen this before).
The final letter of 'mistress' is 's'.
'i'+'s'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "Shares" , "One of a series published periodically" , "Children, edition" , "Topic (to be addressed)" , "Hot topic; outcome" .)