Making sense of a pudding mix (6,2)
I believe the answer is:
Here is my best explanation:
'making sense' is the definition.
('add up' can be a synonym of 'make sense')
'a pudding mix' is the wordplay.
'mix' is an anagram indicator.
'pudding' with letters rearranged gives 'ddingup'.
'of' acts as a link.
(Another definition for adding up that I've seen is "Making sense of".)