Idiot born with a certain effervescence (8)
I believe the answer is:
bonehead
'idiot' is the definition.
(both can mean a stupid person)
'born with a certain effervescence' is the wordplay.
'born' becomes 'b' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'a' becomes 'one'.
'certain effervescence' becomes 'head' (I can't explain this - if you can you should give a lot more credence to this answer).
'b'+'one'+'head'='BONEHEAD'
Can you help me to learn more?
(Other definitions for bonehead that I've seen before include "Stupid person with ossified cranium" , "Idiot" , "Stupid person with ossified brain" , "stupid leader" , "dope" .)