Old priestess finds mantra in book one (9)
I believe the answer is:
bacchante
'old priestess' is the definition.
'bacchante' can be an answer for 'priestess' (I've seen this before). I'm not certain of the 'old' bit.
'mantra in book one' is the wordplay.
'mantra' becomes 'chant' (I've seen this before).
'in' means one lot of letters goes inside another.
'book' becomes 'b' (abbreviation for book).
'one' becomes 'ace' (ace has a value of one in some card games).
'b'+'ace'='bace'
'chant' going inside 'bace' is 'BACCHANTE'.
'finds' acts as a link.
Can you help me to learn more?
(Other definitions for bacchante that I've seen before include "female priest" , "priestess" , "Drunken reveller" .)