Mostly note how he cheated: left to break up (2,3,5)
I believe the answer is:
on the whole
'mostly' is the definition.
(I've seen this in another clue)
'note how he cheated left to break up' is the wordplay.
I cannot quite understand how this works, but
'note' could be 'whole' (whole note is a kind of note) and 'whole' is present in the answer.
'he' is within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for on the whole that I've seen before include "First and last" , "In general" , "Generally" , "All things considered" , "as a rule" .)