Beginning stage (5)
'beginning stage' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for onset that I've seen before include "Start, beginning" , "Inception" , "Storming" , "starting?" , "Beginning or early stages" .)
(Other definitions for germ that I've seen before include "Microorganism causing disease" , "virus?" , "infectious organism" , "Tiny organism that spreads disease" , "From which disease comes" .)