Not by a different study (6)
I believe the answer is:
botany
'study' is the definition.
(botany is a kind of study)
'not by a different' is the wordplay.
'different' is an anagram indicator (letters placed in a different order).
'not'+'by'+'a'='notbya'
'notbya' anagrammed gives 'BOTANY'.
(Other definitions for botany that I've seen before include "Plant study" , "Unlucky, doomed" , "type of wool" , "the name of a bay" , "plants growing in one area" .)