Mirror left amidst vapour (sulphur) (5)
I believe the answer is:
glass
'mirror' is the definition.
(glass is a kind of mirror)
'left amidst vapour sulphur' is the wordplay.
'left' becomes 'l' (common abbreviation).
'amidst' indicates putting letters inside.
'vapour' becomes 'gas' (I've seen this before).
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'l' put into 'gas' is 'glas'.
'glas'+'s'='GLASS'
(Other definitions for glass that I've seen before include "Transparent substance" , "Translucent, usually transparent, material" , "Goblet" , "Brittle transparent material" , "Calm lake may be compared to this" .)