"do not swallow one!"

This is a reversal clue - look for indicators like 'back', 'returned', 'up' (in down clues), or 'west' (in across clues)

Join the wordplay parts together in sequence

Answer: YARN | Definition: 'do not swallow one!' | Wordplay: Y-[e]-AR=”period” minus ‘e’ for ecstasy=”Drug”; plus end of [addictio]-N