A little problem? So we hear (4)
I believe the answer is:
some
'a little' is the definition.
(I've seen this before)
'problem? so we hear' is the wordplay.
'problem?' becomes 'sum' (arithmetic problem).
'so we hear' shows a homophone (sound like).
'sum' is a homophone of 'SOME'.
(Other definitions for some that I've seen before include "Hard work" , "A little; not a little" , "An undetermined number" , "certain" , "Unknown or unspecified amount or number" .)