19/4 down
It
really
is
me
in
motorboat
at
sea!
(2,3,1,5)
It really is me in motorboat at sea! (2,3,1,5)
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The definition is at the beginning of the clue.
How the clue works
Look at It really is me — that's your definition, the kind of thing you'd insist when proving you're human. Then take in motorboat and scramble the letters, with "at sea" doing the work as your anagram indicator, and they rearrange neatly to give you the answer.