Problem not all discussed? (3)
I believe the answer is:
sum
'problem' is the definition.
(arithmetic problem)
'not all discussed?' is the wordplay.
'not all' becomes 'some' ('some' is the opposite to 'all').
'discussed?' indicates a 'sounds like' (homophone) clue (I've seen 'discuss' mean this).
'some' sounds like 'SUM'.
(Other definitions for sum that I've seen before include "See 25" , "Amount of money; add" , "An additional problem" , "Mathematical problem" , "Quantity obtained by addition" .)