It's spread about that mice took a swim back east. (8)
I believe the answer is:
epidemic
'it's spread' is the definition.
(thesaurus)
'that mice took a swim back east' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'mice' is 'emic' which is within the answer.
'east' could be 'e' (synonyms) and 'e' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'about' is the link.
Can you help me to learn more?
(Other definitions for epidemic that I've seen before include "Widespread occurrence of a disease" , "Prevailing" , "lot of disease" , "Fast food item" , "medical emergency" .)