Whole or just part, say (3)
I believe the answer is:
sum
'whole' is the definition.
(sum is a kind of whole)
'part say' is the wordplay.
'part' becomes 'some' (I've seen this before).
'say' indicates a 'sounds like' (homophone) clue.
'some' sounds like 'SUM'.
'or just' is the link.
(Other definitions for sum that I've seen before include "An additional problem" , "See 25" , "Arithmetic calculation" , "Quantity of money" , "Total; problem" .)