Part sounds like a problem (4)
I believe the answer is:
some
'part' is the definition.
(I've seen this before)
'sounds like a problem' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'a problem' becomes 'sum' (arithmetic problem).
'sum' is a homophone of 'SOME'.
(Other definitions for some that I've seen before include "An undetermined number" , "Several" , "Hard work" , "An unspecified quantity, not all" , "Unknown or unspecified amount or number" .)