.... doubter starts saying ... "Certain English people talk in conundrums!" (7)
I believe the answer is:
sceptic
'doubter' is the definition.
(synonyms)
'starts saying certain english people talk in conundrums' is the wordplay.
'starts' suggests taking the first letters.
The first letters of 'saying certain english people talk in conundrums' is 'SCEPTIC'.
(Other definitions for sceptic that I've seen before include "Thomas for one" , "One might doubt it" , "One inclined to question accepted opinions" , "Doubter; atheist" , "One questioning accepted opinions" .)