Mutated rural mice become erratic (9)
I believe the answer is:
mercurial
'erratic' is the definition.
(I know that erratic can be written as mercurial)
'mutated rural mice' is the wordplay.
'mutated' is an anagram indicator.
'rural'+'mice'='ruralmice'
'ruralmice' with letters rearranged gives 'MERCURIAL'.
'become' acts as a link.
(Other definitions for mercurial that I've seen before include "Erratic" , "Changeable in mood" , "Volatile" , "Changing unpredictably" , "Prone to sudden mood swings" .)