A nameless germ spreading disease (6,7)
I believe the answer is:
german measles
'disease' is the definition.
(German measles is an example)
'a nameless germ spreading' is the wordplay.
'spreading' is an anagram indicator.
'a'+'nameless'+'germ'='anamelessgerm'
'anamelessgerm' anagrammed gives 'GERMAN MEASLES'.
(Other definitions for german measles that I've seen before include "complaint" , "Rubella" , "viral illness" .)