Sum to make sense (3,2)
I believe the answer is:
add up
I believe this clue is a double definition.
'sum' is the first definition.
(synonyms)
'make sense' is the second definition.
(synonyms)
'to' acts as a link.
(Other definitions for add up that I've seen before include "Seem consistent or reasonable - arithmetically?" , "Seem to be sensible" , "Sum - make sense" , "Aggregate, get total" , "Total; make sense" .)