Concerning density in burner capacitor (9)
I believe the answer is:
condenser
'capacitor' is the definition.
(synonyms)
'concerning density in burner' is the wordplay.
I cannot quite understand how this works, but
'concerning' could be 'on' (both can mean 'in relation to') and 'on' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for condenser that I've seen before include "cooler?" , "contracts" , "Another name for a capacitor" , "One may have charge" , "type of lens" .)