Open-ended novel involves gifted Marple attacking mystery (6)
I believe the answer is:
enigma
'mystery' is the definition.
(I know that mystery can be written as enigma)
'open-ended novel involves gifted marple attacking' is the wordplay.
'open' says to take the initial letters (take the opening letter).
The first letters of 'ended novel involves gifted marple attacking' is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysterious puzzle" , "cryptic puzzle!" , "'Puzzle, riddle (6)'" , "closed book" , "Elgar Variations nickname" .)