Remove borders from flag outside old section of building (6)
I believe the answer is:
annexe
'section of building' is the definition.
(I know that annexe is part of building)
'remove borders from flag outside old' is the wordplay.
I cannot really understand how this works, but
'from' could be 'ex' ('from' in Latin) and 'ex' is found in the answer.
The remaining letters 'anne' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Side building" , "Extra piece of building" , "Adjoining building" , "Wing of the main building" , "Additional room etc for house" .)