Strap yourself in and shut it! (4,2)
I believe the answer is:
belt up
'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" .)