Eton prepared to change rule (12)
I believe the answer is:
preponderate
'rule' is the definition.
(I've seen this before)
'eton prepared to change' is the wordplay.
'to change' indicates an anagram.
'eton'+'prepared'='etonprepared'
'etonprepared' with letters rearranged gives 'PREPONDERATE'.
(Other definitions for preponderate that I've seen before include "more important" , "rule" .)