Problem causes you harm, always at the end (3)
I believe the answer is:
'problem' is the definition.
'causes you harm always at the end' is the wordplay.
'causes' is an anagram indicator (I've seen 'causing' mean this).
'at the end' suggests the final letters.
The final letters of 'you harm always' are 'ums'.
'ums' is an anagram of 'SUM'.
(Other definitions for sum that I've seen before include "Arithmetical problem" , "Mathematical problem" , "Quantity obtained by addition" , "Aggregate, total" , "An additional problem" .)