Runner has seen germs spread (9)
I believe the answer is:
messenger
'runner' is the definition.
(I've seen this before)
'seen germs spread' is the wordplay.
'spread' is an anagram indicator.
'seen'+'germs'='seengerms'
'seengerms' anagrammed gives 'MESSENGER'.
'has' is the link.
(Other definitions for messenger that I've seen before include "The role of Hermes or Mercury?" , "Delivery boy" , "Emissary, courier" , "Courier - don't shoot him" , "He brings the news - don't shoot him" .)