Idea from Met about to seize heroin and ecstasy (5)
I believe the answer is:
theme
'idea' is the definition.
(theme is a kind of idea)
'met about to seize heroin and ecstasy' is the wordplay.
'about' says the letters should be written backwards.
'to seize' indicates putting letters inside.
'heroin' becomes 'H' (H is a slang term for heroin).
'and' says to put letters next to each other.
'ecstasy' becomes 'E' (informal term for the drug).
'met' written backwards gives 'tem'.
'tem' enclosing 'h' is 'them'.
'them'+'e'='THEME'
'from' is the link.
(Other definitions for theme that I've seen before include "Short melody" , "Thread, melody" , "issue" , "Signature tune" , "Main idea or topic" .)