### Clue: Teacher turning up during autopsy's a possible shedder of light (5)
### Answer: PRISM 
 ### Explanation: Definition: a possible shedder of light
 **Reversal of**(turning up, in a down clue) SIR(term of address for a school teacher) **contained in**(during) P.M.(an autopsy, from the abbrev. for "post-mortem").
Defn: … or a refractor of light.