Defeated by poorly-designed antechamber with absolutely no charm (6)
I believe the answer is:
beaten
'defeated' is the definition.
(beating is a kind of defeating)
'by poorly-designed antechamber with absolutely no charm' is the wordplay.
I cannot really see how this works, but
'by' could be 'at' and 'at' is present in the answer.
'no' could be 'n' (common abbreviation) and 'n' is found within the answer.
The remaining letters 'bee' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for beaten that I've seen before include "Failed to win - punished" , "Hammered" , "Formed or made thin by hammering" , "Vanquished" , "Struck; defeated" .)