A disease found by one rodent (6)
I believe the answer is:
agouti
'rodent' is the definition.
(agouti is a kind of rodent)
'a disease found by one' is the wordplay.
'disease' becomes 'gout' (gout is a kind of disease**).
'found by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'a'+'gout'+'i'='AGOUTI'
(Other definitions for agouti that I've seen before include "Creature" , "Large rodent" , "South American rodent" , "S.Amer. rodent" , "Guinea pig-like rodent" .)