Lad penning books to get prize (5)
I believe the answer is:
booty
'prize' is the definition.
(I know that prize can be written as booty)
'lad penning books' is the wordplay.
'lad' becomes 'boy' (I've seen this in another clue).
'penning' means one lot of letters goes inside another.
'books' becomes 'OT' (Old Testament).
'boy' going around 'ot' is 'BOOTY'.
'to get' is the link.
(Other definitions for booty that I've seen before include "Spoils" , "Spoil - plunder" , "Loot" , "Ill-gotten gains" , "Swag" .)