Most successful pledge disposing of Bible as a gift (8)
I believe the answer is:
bestowal
'a gift' is the definition.
(bestowal is a kind of gift)
'most successful pledge disposing of bible' is the wordplay.
'most successful' becomes 'best'.
'pledge' becomes 'avowal'.
'disposing of' is a deletion indicator.
'bible' becomes 'av'.
'avowal' with 'av' taken away is 'owal'.
'best'+'owal'='BESTOWAL'
'as' acts as a link.
(Other definitions for bestowal that I've seen before include "Act of giving" , "award" , "Act of conferring (eg, grant on someone)" , "disposal" , "Conferment (of an honour)" .)