Den crawling with mice found locally (7)
I believe the answer is:
endemic
'found locally' is the definition.
(something endemic is found in a certain place)
'den crawling with mice' is the wordplay.
'crawling with' is an anagram indicator (letters crawl around).
'den' anagrammed with 'mice' can make 'ENDEMIC'.
(Other definitions for endemic that I've seen before include "(Of a disease) regularly found in a certain area" , "Local (of disease)" , "Confined to a particular area" , "constantly present in the locality" , "Prevalent among a group of people, disease perhaps" .)