Promising biographer's first title (6)
I believe the answer is:
bright
'promising' is the definition.
(I know that promising can be written as bright)
'biographer's first title' is the wordplay.
'biographer's first' becomes 'b' (1st letter of 'biographer').
'title' becomes 'right' (I've seen this before).
'b'+'right'='BRIGHT'
(Other definitions for bright that I've seen before include "bubbly?" , "Lustrous, shining" , "Clever - dazzling" , "Shining (with intelligence)" , "Clever; fine" .)