Authentic sort of earl (4)
I believe the answer is:
real
'authentic' is the definition.
(authentic means real or genuine)
'sort of earl' is the wordplay.
'sort of' is an anagram indicator.
'earl' with letters rearranged gives 'REAL'.
(Other definitions for real that I've seen before include "Unaffected" , "Substantial" , "Physically existing" , "Lear, strangely, could actually exist" , "Type of ale" .)