Warning gold is going down in sale (7)
I believe the answer is:
caution
'warning' is the definition.
(as in a police caution)
'gold is going down in sale' is the wordplay.
I cannot quite see how this works, but
'gold' could be 'au' (Au is the chemical symbol for gold) and 'au' is found in the answer.
'going' could be 'on' (both can mean functioning or activated) and 'on' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for caution that I've seen before include "Carefulness - yellow card?" , "Care; police warning" , "amusing type" , "Warn strongly" , "Hoot" .)