9 across
Seriously
funny
minute
where
head
blown
off
in
which
Groucho’s
prop
backfires?
(10)
Seriously funny minute where head blown off in which Groucho’s prop backfires? (10)
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The definition is at the beginning of the clue.
How the clue works
The definition here is Seriously funny, which describes something that's both tragic and comic at once. You start with ATOMIC meaning "minute", chop off the first letter ("head blown off") to get TOMIC, then reverse CIGAR (Groucho Marx's famous prop, "backfires") to get RAGIC, and wrap TOMIC around that reversed CIGAR to build the full answer.