Scientist came across a lot of magnetism with substance (12)
I believe the answer is:
metallurgist
'scientist' is the definition.
(I've seen this before)
'came across a lot of magnetism with substance' is the wordplay.
'came across' becomes 'met' (to come across is to meet or encounter).
'a lot of' means to remove the last letter (most of the word but not all of it).
'magnetism' becomes 'allure' (I've seen this before).
'with' says to put letters next to each other.
'substance' becomes 'gist' (synonyms).
'allure' with its final letter taken away is 'allur'.
'met'+'allur'+'gist'='METALLURGIST'
(Other definitions for metallurgist that I've seen before include "Studier of certain elements or alloys" , "Scientist" .)