25 across
I
made
a
mistake
taking
heroin
during
journey
–
19s
might
be
on
it
(9)
I made a mistake taking heroin during journey – 19s might be on it (9)
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The definition is at the end of the clue.
How the clue works
The definition is 19s might be on it — where 19 refers to the answer to 19 down, which is DRAFTEE, and draftees (conscripts) are exactly the sort of people you'd find on a troopship. For the wordplay, take OOPS ("I made a mistake") and H (heroin), and tuck them both inside TRIP (journey): TR, then OOPS, then H, then IP gives you the whole thing.