15 across
Dreary
lecture,
reportedly
–
that
would
never
take
off
(5,3)
Dreary lecture, reportedly – that would never take off (5,3)
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The definition is at the end of the clue.
How the clue works
Look at that would never take off — that's your definition, pointing to the famously flightless (and very much extinct) Great Auk. The wordplay builds "grey talk" from dreary (grey) and lecture (talk), and then reportedly tells you to say it aloud — "grey talk" sounds just like GREAT AUK.