Your legs collapsed, climbing the stairs? That's unimaginable! (3,7,4,2,2)

'stairs? that's unimaginable' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'your legs collapsed climbing the' is the wordplay.
I cannot really understand how this works, but
'climbing' could be 'up' and 'up' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?