Train up to be a moralist (7)
I believe the answer is:
puritan
'a moralist' is the definition.
(I've seen this before)
'train up to be' is the wordplay.
'to be' indicates an anagram.
'train'+'up'='trainup'
'trainup' with letters rearranged gives 'PURITAN'.
(Other definitions for puritan that I've seen before include "One with very strict moral principles" , "Adherent ot strict religous principles, opposed to sensual pleasures" , "Strict person for whom pleasure is sinful" , "very proper person?" , "Person of extreme moral strictness" .)