Retire from the witness box: the structure has fallen (5,4)
I believe the answer is:
stand down
'retire' is the definition.
(thesaurus)
'witness box the structure has fallen' is the wordplay.
I cannot quite understand how this works, but
'structure' could be 'stand' (stand is a kind of structure) 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).
'from the' is the link.
Can you help me to learn more?
(Other definitions for stand down that I've seen before include "Relinquish one's post" , "not to be a candidate" , "Resign (from office)" , "Withdraw as from competition, be seated" , "Exhibition equipment packed up" .)