Drug that's popular filling plate beside me (9)
I believe the answer is:
mescaline
'drug' is the definition.
(mescaline is a kind of drug)
'popular filling plate beside me' is the wordplay.
'popular' becomes 'in'.
'filling' means one lot of letters goes inside another.
'plate' becomes 'scale' (synonyms).
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'in' placed inside 'scale' is 'scaline'.
'scaline' after 'me' is 'MESCALINE'.
'that's' is the link.
(Other definitions for mescaline that I've seen before include "Drug" , "Cactus hallucinogen" .)