Rod is an idiot (8)
I believe the answer is:
dipstick
This clue is a double definition.
'rod' is the first definition.
(a rod used to measure the depth of a liquid)
'an idiot' is the second definition.
(informal term for a stupid person)
'is' is the link.
(Other definitions for dipstick that I've seen before include "Stupid person" , "Turkey" , "Engine oil measurer" , "Fluid measuring rod" , "Rod for measuring depth of liquid contents" .)