Breaking up is great (8)
I believe the answer is:
cracking
I believe this is a double definition.
'breaking up' is the first definition.
('crack' can be a synonym of 'break up')
'great' is the second definition.
('cracking' can be a synonym of 'great')
'is' is the link.
(Other definitions for cracking that I've seen before include "very fast" , "Wonderful" , "Excellent; fracturing" , "failing" , "great!" .)