Rod-shaped bacterium (8)
I believe the answer is:
bacillus
'rod-shaped bacterium' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bacillus that I've seen before include "CIA bulls, oddly, have a kind of germ" , "Type of bacterium" , "Rod-like bacterium, may cause disease" , "Rod-like bacterium found in lilac bus" , "Single-cell organism, can cause illness" .)