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Critical
report
is,
theoretically,
what
started
it
all
(3,4)
Critical report is, theoretically, what started it all (3,4)
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The definition is at the end of the clue.
How the clue works
The definition is what started it all, which refers to the theoretical origin of the universe, the BIG BANG. In the wordplay, 'critical' gives BIG (as in important) and 'report' gives BANG (a loud noise like a gunshot), and the two are placed side by side.