Adviser has more trouble with books (6)
I believe the answer is:
'adviser' is the definition.
(I've seen this before)
'more trouble with books' is the wordplay.
'trouble' is an anagram indicator.
'with' indicates putting letters inside.
'books' becomes 'NT' (New Testament).
'more' with letters rearranged gives 'meor'.
'meor' going around 'nt' is 'MENTOR'.
'has' acts as a link.
(Other definitions for mentor that I've seen before include "Personal guide" , "Counsellor/guide" , "Truster advisor" , "One guiding" , "Experienced adviser" .)