Set gold to same prefix (4)
I believe the answer is:
auto
'same prefix' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'set gold to' is the wordplay.
'set gold' becomes 'au' (Au is the chemical symbol for gold. I am not sure about the 'set' bit.).
'au'+'to'='AUTO'
Can you help me to learn more?
(Other definitions for auto that I've seen before include "Car (American)" , "Motor vehicle, in short" , "American vehicle" , "Car, in short" , "Car (chiefly in US)" .)