Assume this person’s opposed to MDMA (7)
I believe the answer is:
imagine
'assume' is the definition.
(I've seen this before)
'this person's opposed to mdma' is the wordplay.
'this person's' becomes 'I'm' (('I am' - 'this person' means the crossword setter).
'opposed to' becomes 'agin' (dialect form of against).
'mdma' becomes 'E' (MDMA is ecstasy, ie 'E').
'im'+'agin'+'e'='IMAGINE'
(Other definitions for imagine that I've seen before include "Picture in mind, envisage" , "Conceive; fancy" , "Suppose; conceive" , "John Lennon hit" , "John Lennon classic (just think of it!)" .)