Imagines content being rewritten to create a mystery (6)
I believe the answer is:
enigma
'a mystery' is the definition.
(I know that mystery can be written as enigma)
'imagines content being rewritten' is the wordplay.
'content' means to look at the middle letters.
'being rewritten' indicates an anagram.
The central letters of 'imagines' are 'magine'.
'magine' is an anagram of 'ENIGMA'.
'to create' acts as a link.
(Other definitions for enigma that I've seen before include "German military cipher machine" , "Puzzle for gaming" , "Title of Elgar's set of variations" , "WWII cipher" , "Gamine being a bit of a puzzle" .)