Problem in a gem being reset (6)
I believe the answer is:
enigma
'problem' is the definition.
(I know that enigma is a type of problem)
'in a gem being reset' is the wordplay.
'being reset' indicates anagramming the letters.
'in'+'a'+'gem'='inagem'
'inagem' anagrammed gives 'ENIGMA'.
(Other definitions for enigma that I've seen before include "It's mysterious like gamine" , "Gamine being a bit of a puzzle" , "'Puzzle, riddle (6)'" , "'Mystery, riddle (6)'" , "Puzzle of gamine (6)," .)