Question mark surrounds top-class book (5)
I believe the answer is:
doubt
'question' is the definition.
(I know that question can be written as doubt)
'mark surrounds top-class book' is the wordplay.
'mark' becomes 'dot' (dotting is a kind of marking).
'surrounds' is an insertion indicator.
'top-class' becomes 'U' (as in U and non-U speech).
'book' becomes 'b' (abbreviation for book).
'u'+'b'='ub'
'dot' going around 'ub' is 'DOUBT'.
(Other definitions for doubt that I've seen before include "One certainly can't have it" , "Feeling of uncertainty about something" , "Suspicion" , "Misgiving" , "Be uncertain about or consider unlikely" .)