Promising book fair (6)
I believe the answer is:
bright
'promising' is the definition.
(I know that promising can be written as bright)
'book fair' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'fair' becomes 'right'.
'b'+'right'='BRIGHT'
(Other definitions for bright that I've seen before include "Illuminated" , "Jolly" , "Vivid; cheerfully lively" , "Clever; vivid" , "Sparkling" .)