Leave the witness box: the platform has collapsed (5,4)
I believe the answer is:
stand down
'leave the witness box' is the definition.
(I've seen this before)
'platform has collapsed' is the wordplay.
I cannot really understand how this works, but
'platform' could be 'stand' (stand is a kind of platform) and 'stand' is found in the answer.
The remaining letters 'down' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for stand down that I've seen before include "Quit" , "Withdraw voluntarily, from job or competition" , "Resign (from office)" , "Withdraw as from competition, be seated" , "Exhibition equipment packed up" .)