Crossword Genius

Strap yourself in and shut it! (4,2)

Ross

I believe the answer is:

belt up

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'and shut it' is the definition.
I can't judge whether this defines the answer.

'strap yourself' is the wordplay.
'strap' becomes 'belt' (I've seen this before).
'yourself' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'belt'+'up'='BELT UP'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for belt up that I've seen before include "Be quiet (colloq.)" , "Rudely silence" , "Be quiet, or secure yourself in car seat" , "Strap yourself in - keep quiet!" , "Informally, stop speaking - use waist band" .)

I've seen this clue in The Telegraph.
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