Idea from Met about to seize heroin and ecstasy (5)
I believe the answer is:
theme
'idea' is the definition.
(I know that idea can be written as theme)
'met about to seize heroin and ecstasy' is the wordplay.
'about' shows that the letters should be reversed in order.
'to seize' means one lot of letters goes inside another.
'heroin' becomes 'H' (H is a slang term for heroin).
'and' is a charade indicator (letters next to each other).
'ecstasy' becomes 'E' (informal term for the drug).
'met' back-to-front is 'tem'.
'tem' placed around 'h' is 'them'.
'them'+'e'='THEME'
'from' acts as a link.
(Other definitions for theme that I've seen before include "Short melody" , "Thread, melody" , "issue" , "Signature tune" , "Main idea or topic" .)