A demon barber and son quarrelling (2,4)
I believe the answer is:
at odds
'quarrelling' is the definition.
(I've seen this before)
'a demon barber and son' is the wordplay.
'demon barber' becomes 'todd' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'son' becomes 's' (genealogical abbreviation for son).
'a'+'todd'+'s'='AT ODDS'
Can you help me to learn more?
(Other definitions for at odds that I've seen before include "In disagreement or dispute" , "On bad terms" , "Not in agreement" , "Quarrelling" , "In opposition or disagreement" .)