Old boy and two blokes rock (8)
I believe the answer is:
obsidian
'rock' is the definition.
(obsidian is a kind of rock)
'old boy and two blokes' is the wordplay.
'two blokes' means two replacements for 'bloke'.
'old boy' becomes 'ob' (abbreviation).
'and' means one lot of letters go next to another.
'bloke' becomes 'sid' (I am not sure about this - if you are sure you should believe this answer much more).
'bloke' becomes 'ian' (man's name).
'ob'+'sid'+'ian'='OBSIDIAN'
Can you help me to learn more?
(Other definitions for obsidian that I've seen before include "Dark volcanic rock" , "A glassy volcanic rock" , "Dark glassy rock formed from lava" , "Glass bottle may be in this" , "Dark volcanic glass" .)