Come down and stay put, idiot! (8)
I believe the answer is:
dipstick
'idiot' is the definition.
(informal term for a stupid person)
'come down and stay' is the wordplay.
'come down' becomes 'dip' (I am not sure about this - if you are sure you should believe this answer much more).
'and' says to put letters next to each other.
'stay' becomes 'stick' (synonyms).
'dip'+'stick'='DIPSTICK'
'put' is the link.
Can you help me to learn more?
(Other definitions for dipstick that I've seen before include "Rod used to measure oil level in car" , "Idiot" , "Engine oil measuring device" , "Rod for measuring depth of liquid contents" , "Wally" .)