When nothing legally can be done about one’s dissembling, row! (4,3)
I believe the answer is:
dies non
'when nothing legally can be done' is the definition.
(day on which legal business does not take place)
'about one's dissembling row' is the wordplay.
'about' means one lot of letters goes inside another.
'dissembling' indicates an anagram.
'row' becomes 'din' (both can mean a loud disturbance).
'ones' is an anagram of 'esno'.
'esno' going into 'din' is 'DIES NON'.
(Other definitions for dies non that I've seen before include "when business cannot be transacted" , "Day for nothing" , "no noise in chancery?" .)