Stop mutated mice becoming widespread (7)
I believe the answer is:
endemic
'widespread' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'stop mutated mice' is the wordplay.
'stop' becomes 'end' (synonyms).
'mutated' is an anagram indicator.
'mice' with letters rearranged gives 'emic'.
'end'+'emic'='ENDEMIC'
'becoming' is the link.
Can you help me to learn more?
(Other definitions for endemic that I've seen before include "Restricted to a certain area" , "(Of a disease) prevalent in a group or area" , "Prevalent in a district - dice men (anag)" , "Local (of disease)" , "Commonly found in particular place - may be disease" .)