Isolate mutant rat genes (8)
I believe the answer is:
estrange
'isolate' is the definition.
(thesaurus)
'mutant rat genes' is the wordplay.
'mutant' indicates anagramming the letters.
'rat'+'genes'='ratgenes'
'ratgenes' is an anagram of 'ESTRANGE'.
(Other definitions for estrange that I've seen before include "drive apart" , "Drive away" , "Get on the wrong side of" , "Arouse hostility" , "Ten gears (anag.)" .)