Famous fellow holding alien back (5)
I believe the answer is:
noted
'famous' is the definition.
(noted can mean well-known or famous)
'fellow holding alien back' is the wordplay.
'fellow' becomes 'don' (both can mean an academic).
'holding' is an insertion indicator.
'alien' becomes 'et' (extraterrestrial).
'back' says the letters should be written backwards.
'don' going around 'et' is 'deton'.
'deton' backwards is 'NOTED'.
(Other definitions for noted that I've seen before include "Renowned" , "Significant" , "Famous or eminent" , "Made a record - celebrated" , "Remarkable" .)