Breaking up is commonly so wonderful (8)
I believe the answer is:
smashing
This clue could be a double definition.
'breaking up' is the first definition.
(I've seen this before)
'commonly so wonderful' is the second definition.
'smashing' can be an answer for 'wonderful' (I've seen this before). I'm unsure of the 'commonly so' bit.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for smashing that I've seen before include "Committing act of vandalism" , "Breaking violently into pieces" , "Great" , "2 [COOL] or 3 [DANDY] perhaps, possibly 20d [WICKED] or 22 [GEAR] !" , "ace" .)