At the start of October, genius reorganised classification for rock (7)
I believe the answer is:
igneous
'for rock' is the definition.
(igneous rock is a kind of rock)
'at the start of october genius reorganised' is the wordplay.
'at the start of' indicates taking the first letters (I've seen 'at the start' mean this).
'reorganised' is an anagram indicator.
The initial letter of 'october' is 'o'.
'o'+'genius'='ogenius'
'ogenius' anagrammed gives 'IGNEOUS'.
'classification' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for igneous that I've seen before include "Like fire - type of rock" , "(Rock) of volcanic origin" , "(Of rock) solidified from a molten state" , "Of rocks, volcanic" , "Fire-formed rock" .)