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' says the letters should be written backwards.
'to seize' is an insertion indicator.
'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' written backwards gives '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" .)