Rig on a device must stop short of blockhead (9)
I believe the answer is:
ignoramus
'rig' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'device must stop short of blockhead' is the wordplay.
I cannot really understand how this works, but
'device' could be 'ram' (RAM is an example) and 'ram' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...
'on a' acts as a link.
Can you help me to learn more?
(Other definitions for ignoramus that I've seen before include "I wouldn't know" , "One could be blissful it's said" , "Uneducated person" , "Idiot" , "Dimwit" .)