Herbert trained new monks (8)
I believe the answer is:
brethren
'monks' is the definition.
(I've seen this before)
'herbert trained new' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'herbert' with letters rearranged gives 'brethre'.
'brethre'+'n'='BRETHREN'
(Other definitions for brethren that I've seen before include "Kindred" , "Siblings (archaic)" , "old relatives?" , "Biblical or other old brothers" , "Fellow members of a sect" .)