To ponder about most of the bible is essential (4-4)
I believe the answer is:
must-have
'essential' is the definition.
(I've seen this before)
'to ponder about most of the bible' is the wordplay.
'to ponder' becomes 'muse' ('muse' can be a synonym of 'ponder').
'about' means one lot of letters goes inside another.
'most of' means to remove the last letter.
'bible' becomes 'av' (**).
'the' with its last letter removed is 'th'.
'th'+'av'='thav'
'muse' placed around 'thav' is 'MUST-HAVE'.
'is' acts as a link.
(Other definitions for must-have that I've seen before include "Highly desirable item" , "Essential" .)