Record I had found on unusual mice with plague (8)
I believe the answer is:
epidemic
'plague' is the definition.
(I've seen this before)
'record i had found on unusual mice' is the wordplay.
'record' becomes 'EP' (abbreviation for extended play record).
'i had' becomes 'i'd' (contraction).
'found on' means one lot of letters go next to another.
'unusual' is an anagram indicator.
'mice' anagrammed gives 'emic'.
'ep'+'id'+'emic'='EPIDEMIC'
'with' is the link.
(Other definitions for epidemic that I've seen before include "Disease affecting many in a population at the same time" , "Disease affecting many people simultaneously" , "Outbreak of disease which spreads rapidly" , "Citrus fruit" , "Dim piece (anag)" .)