Germs seen to be spread by a carrier (9)
I believe the answer is:
messenger
'a carrier' is the definition.
(thesaurus)
'germs seen to be spread' is the wordplay.
'to be spread' indicates anagramming the letters.
'germs'+'seen'='germsseen'
'germsseen' is an anagram of 'MESSENGER'.
'by' acts as a link.
(Other definitions for messenger that I've seen before include "Runner" , "Bearer of communication" , "Communication carrier" , "The role of Hermes or Mercury?" , "One who bears a communication" .)