Doctor appears in put-up travel document as writer (6)
I believe the answer is:
asimov
'writer' is the definition.
(Isaac Asimov)
'doctor appears in put-up travel document' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'appears in' indicates putting letters inside.
'put up' says the letters should be written backwards.
'travel document' becomes 'visa' (I've seen this before).
'visa' backwards is 'asiv'.
'mo' put inside 'asiv' is 'ASIMOV'.
'as' acts as a link.
(Other definitions for asimov that I've seen before include "Author of "Three Laws of Robotics"" , "Isaac ---, SF author" , "Isaac --, sci-fi author" , "See 22 Down" , "Isaac --, science fiction author" .)