Irregular noun is all together (6)
I believe the answer is:
unison
'all together' is the definition.
'unison' can be an answer for 'together' (thesaurus). I am not sure about the 'all' bit.
'irregular noun is' is the wordplay.
'irregular' indicates anagramming the letters.
'noun'+'is'='nounis'
'nounis' anagrammed gives 'UNISON'.
Can you help me to learn more?
(Other definitions for unison that I've seen before include "All singing the same note" , "Concord; with same pitch" , "Concord; a trade union" , "There's agreement" , "Trade union (where all agree?)" .)