Run post without slouching? (4,7)

I believe the answer is:

bolt upright

I'm a little stuck... Click here to teach me more about this clue!

'without slouching?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'run post' is the wordplay.
'run' becomes 'bolt' (I've seen this in another clue).
'post' becomes 'upright' (I've seen this before).
'bolt'+'upright'='BOLT UPRIGHT'

Can you help me to learn more?

(Other definitions for bolt upright that I've seen before include "Rigidly erect" , "Definitely not slouching" , "Standing stiffly" , "In alert mode" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.