Mystery or history (5)
I believe the answer is:
genre
'mystery or history' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for genre that I've seen before include "Particular style of art or literature, may be green" , "Green (anag)" , "Type or style, of art" , "class" , "Literary or artistic type or style" .)