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