Part or whole read aloud (4)
I believe the answer is:
some
'part' is the definition.
(thesaurus)
'whole read aloud' is the wordplay.
'whole' becomes 'sum' (sum is a kind of whole).
'read aloud' shows a homophone (sound like).
'sum' sounds like 'SOME'.
'or' is the link.
(Other definitions for some that I've seen before include "Undefined number" , "Unknown or unspecified amount or number" , "certain" , "Part" , "A little; not a little" .)