Celebrity starts to stupidly tell Doctor Doctor gag, essentially, backwards (7)
I believe the answer is:
stardom
'celebrity' is the definition.
(thesaurus)
'doctor gag essentially backwards' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'gag essentially' becomes 'drats' (I am not sure about this - if you are sure you should believe this answer much more).
'backwards' shows that the letters should be reversed in order.
'mo'+'drats'='modrats'
'modrats' written backwards gives 'STARDOM'.
'starts to stupidly tell doctor' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stardom that I've seen before include "Great celebrity" , "wide renown" , "(The prestige of) celebrity" , "Great fame" , "Stage or screen fame" .)