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