Reserves the right to carry book (6)
I believe the answer is:
esther
'book' is the definition.
(book of the Hebrew Bible)
'reserves the right to carry' is the wordplay.
'to carry' indicates the answer is hidden within the clue (I've seen 'carries' mean this).
'ESTHER' is hidden within 'reserves the right'.
(Other definitions for esther that I've seen before include "religious book" , "10 Heroine" , "Good book" , "Old Testament queen" , "There's a girl from the Old Testament" .)