A glass, old boy, is to be raised to Diana shortly (8)
I believe the answer is:
obsidian
'a glass' is the definition.
(I've seen this before)
'old boy is to be raised to diana shortly' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'to be raised' says the letters should be written in reverse (going up in a down clue).
'shortly' means to remove the last letter.
'is' reversed gives 'si'.
'diana' with its final letter removed is 'dian'.
'ob'+'si'+'dian'='OBSIDIAN'
(Other definitions for obsidian that I've seen before include "Dark volcanic rock" , "Dark volcanic glass" , "Volcanic glass-like rock" , "Dark glass-like rock" , "A hard black volcanic glass" .)