A certain additional problem is spoken of (4)
I believe the answer is:
some
'a certain' is the definition.
(eg some person means a certain person)
'problem is spoken of' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'is spoken of' indicates a 'sounds like' (homophone) clue.
'sum' sounds like 'SOME'.
'additional' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for some that I've seen before include "Unknown or unspecified amount or number" , "A little; not a little" , "An undetermined number" , "Part" , "An unspecified quantity, not all" .)