Rod takes the plunge a short time later? (8)
I believe the answer is:
dipstick
'rod' is the definition.
(a rod used to measure the depth of a liquid)
'the plunge a short time later?' is the wordplay.
'the plunge' becomes 'dip' ('dip' can be a synonym of 'plunge').
'a short time later?' becomes 'stick' (I can't justify this - if you can you should give a lot more credence to this answer).
'dip'+'stick'='DIPSTICK'
'takes' is the link.
Can you help me to learn more?
(Other definitions for dipstick that I've seen before include "Idiot" , "Rod for measuring depth of liquid contents" , "Engine oil measuring device" , "Rod for measuring oil in engine" , "Engine oil measurer" .)