Notice eccentric concealing popular drug (8)
I believe the answer is:
nicotine
'drug' is the definition.
(I know that Nicotine is a drug)
'notice eccentric concealing popular' is the wordplay.
'eccentric' is an anagram indicator.
'concealing' means one lot of letters goes inside another.
'popular' becomes 'in' ('in' can mean trendy).
'notice' is an anagram of 'nicote'.
'nicote' going around 'in' is 'NICOTINE'.
(Other definitions for nicotine that I've seen before include "Nervous" , "Tobacco alkaloid" , "Chief constituent of tobacco" , "Tobacco element" , "Tobacco toxin" .)