Jonathan Ross's expert declared to be a charlatan (5)
I believe the answer is:
quack
'charlatan' is the definition.
(quack is a kind of charlatan)
I can't explain the rest of the clue.
(Other definitions for quack that I've seen before include "Bogus doctor" , "Pseudo doctor" , "phoney" , "Mountebank medic" , "Duck's cry or dud doctor" .)