Joy's rear reflected in mirror (8)
I believe the answer is:
gladness
'joy's' is the definition.
(I've seen this before)
'rear reflected in mirror' is the wordplay.
'rear' becomes 'end' (I've seen this in other clues).
'reflected' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'mirror' becomes 'glass' (glass is a kind of mirror**).
'end' in reverse letter order is 'dne'.
'dne' going into 'glass' is 'GLADNESS'.
(Other definitions for gladness that I've seen before include "Happy state" , "Pleasure" , "Joy" , "delight" .)