Old rock band is one for rock (8)
I believe the answer is:
obsidian
'for rock' is the definition.
(obsidian is a kind of rock)
'old rock band is one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rock' is an anagram indicator (the solver must 'rock' the letters around).
'one' becomes 'i' (Roman numeral).
'band'+'is'+'i'='bandisi'
'bandisi' with letters rearranged gives 'bsidian'.
'o'+'bsidian'='OBSIDIAN'
(Other definitions for obsidian that I've seen before include "Dark glassy rock formed from lava" , "Dark glass-like rock" , "Dark volcanic glass" , "A hard black volcanic glass" , "Glass bottle may be in this" .)